It's obvious that a player will choose classes naturally apt towards something to fill that very role, but I believe the levels of that aptitude and the difference they make in practice were not designed that well.
In MM 3 to 6, hybrid classes such as the Archer and the Paladin had access to the entire library of clerical and arcane spells. To compensate, they had less HP than pure knights and less SP than pure spellcasters. And I believe that was well enough to make them worse at magic than pure spellcasters. The Druid suffered terribly in MM3-5 though, being given a spell library combining the Clerical and the Arcane, but without the really good spells in it (Walk on Water being an exception). But though a hybrid, the Druid was still a pure spellcaster - and its hybrid counterpart the Ranger still had full access to all the Druidic spells all the same.
I believe the Druid was at his finest in MM6, where he got full access to all six elemental and self schools, being denied only the Light and Dark realm, which the most powerful spells went to. A fair deal by all means IMO - if you had a Sorcerer and a Druid or a Cleric and a Druid, you basically didn't need the other specialized spellcaster in your party, since you had more value with the Druid (unless you were very fond of having 2 Shrapmetal casters, which is well, understandable

But then MM7 came with its class-restrictive system, which as I said was not a bad idea per se, but I believe they could have thought a bit more about it. Here's why.
Having access to GM Light or Dark is reason enough to want a Cleric or a Sorcerer in your party, that's for sure. Only they can learn those schools and use them to any good effect. And it's fine, it's the way things should be. But the problem is, MM7 introduced the Grandmaster level, which with many schools of magic unfortunately makes too much of a difference. This shines through the most in case of Body Magic - Power Cure is a wondrous spell, as is the ability do resist death and eradication granted by GM Protection of Magic. These spells are just too good ad GM and too weak at plain M. It is also essential to GM Water Magic for Lloyd's Beacon, Air Magic not to have to worry about your Invisibility running out (Starburst is also nice), Fire Magic to even have access to a decently powered and safe offensive spell etc. Funny thing is, Light and Dark Magic start being sufficiently awesome at Master level and GM is just a luxury perk, but the only classes that can go above Normal get GM by default anyway, so it's quite hard to appreciate this. Add the fact of not being able to learn spell books for spells above your level also brought with this game, and suddenly clerics and sorcerers become so much better than druids that it's not worth even bothering with the latter.
I mean, what's a Druid supposed to be? A Sorcerer/Cleric cross. And what is it in practice? A Paladin/Archer cross, minus all the martial proficiency. I mean come on, the Druid is below the Cleric but above the Sorcerer in HP, weapon and armor selection. The Alchemy GM is very nice, but it alone isn't enough to make it worth keeping him around, not by a long shot. GM Meditation? And what's the Druid gonna be using that massive, massive SP pool on? Implosions? Acid bursts? The fact that the Druid can't GM the elemental and Self schools is utterly ridiculous. He NEEDS Incinerate. He NEEDS Power Cure. He NEEDS Resurrection. Otherwise he's just a joke.
The sad thing with the current system is: once you go Cleric, you never go back (which generally also holds true for the Sorcerer), which therefore greatly limits team building diversity for a successful, fun-to-play team. You lack a Cleric or a Sorcerer, you're gonna have a hard time (or at least "comparatively hard"), and not really because of Light and Dark magic. Aside from these two schools though, who would want to sacrifice GM Protection from Magic or Lloyd's Beacon? IMO this matter would have been rectified somewhat if the previous spell system had been kept - being able to learn any spell at any level, with four levels of proficiency, or at least needing only Expert for the five strongest spells. Obviously we can't have that, and even the game lore contradicts this somewhat.
But there is another, even better way.
Bump up all the hybrid classes' available magic skill levels one notch.
I mean, will it make the classes overpowered? Absolutely not - their mana pools, along with the Sorcerers/Clerics sole full Light/Dark prowess will still restrict them. Also, it will give the player bigger freedom of choice with some classes, letting them really make a statement with their magic and enabling the player to have proper fun with the hybrid classes.
Thief - elevate magic skills from Normal to Expert, the latter accessible after second promotion. The thief still has quite a bit of mana at higher levels - does anyone even bother using it on those puny Firebolts? Didn't think so. Basic Meditation could also be learnable after second promotion. This would allow the Thief to perhaps specialize and get at least one decent ranged attack of choice (Fireball, Blades etc.)
Monk - Allow Ninjas to cast Expert Self Magic just as Masters can. To compensate, give Masters basic Meditation and Merchant and Expert in Bow (the "archer monk" is quite a popular trope after all!). Why all of this? Well, Ninjas need Hammerhands too, the spell was made for this very class!
Druid - Give him GM in all the schools he can learn, dammit! Expert in Staff would also be cool, not that Druids use it anyway. Finally his vast mana pool will not be wasted. So much.
Paladin - GM in Self Magic, Expert in Light/Dark. Up to Expert learnable right off the bat, one more level of mastery per promotion. Light/Dark learnable at Crusader, Expert at final promotion. Why? Because Normal is pointless, good as those spells are, and Expert - while better - is still one level short of the really good and worthwhile spells you get at Master.
Archer - Analogous to the Paladin. Remove Spear Master, because there is nothing to dual wield the spear with anyway (Dagger is not a viable choice due to the broken damage formula I made a thread about earlier).
Ranger - Master in all schools. This would make the class really the jack-of-all-trades, rather than the jack-of-all-shit it is now. Up to Expert at Hunter, Mastery after the final promotion. You're asking why? Well, the Ranger has just as much mana as the Paladin and the Archer do. Why should he waste it all on Acid Bursts if those two classes can get Master spells? Because then those two classes' existence would have no point, you say. Well, now they have Grand in their schools of choice, and so do Druids, so it's only fair! Running 4 Rangers would actually not be such an exercise in masochism now.
Cleric - Keep the magic schools as they are, except let the class learn Light/Dark magic at Priest, up to Expert, after which a promotion would be needed. Buff Staff to Expert (just like it is in MM8), add basic Armsmaster skill (also as per MM8).
Sorcerer - Do the same Light/Dark gimmick as with the Cleric. Alchemy and Meditation don't need buffs, since the Druid is actually useful now and Alchemy would be a nice cherry on top of the cake for him. Meditation COULD be buffed to Grand, though, as it is that way in MM8. Sorcerers kinda need the extra mana too.
So, what do you think about these changes? I've seen several skill table mods already, but they were more elaborate and dare I say, gimmicky (like letting a class GM only one of the elemental schools etc). This is more of a complete overhaul but also an evened out improvement.