I think the ability to have more available options in combat leads to greater variance of play and requires better real time decision making.
The ability to adapt to changing situations in the fight is better than setting up a plan (or copy/pasting what someone else said was the best way to do it) before the fight and being stuck with that limited set of options.
I could just as easily argue that having all available abilities and spells available at all times makes for cookie cutter, homogonized gameplay because then every person playing every one of those classes at that same moment has all the same options and it's just simply a matter of copying what the 'right' button press would be in that moment. If you have all abilities available, then there's one right button to press any given moment. If you have only a fraction of them available, then you have to look at the tools that are available and make a decision from there, and your tools may be different from someone of the same class in that same moment the day before, thus you're differentiated.
The point being, some people like all available spells/abilities/skills, some people don't. There are good reasons for and against it. I find your reasoning a bit flawed, though, because it can be used to either support or dismiss both options.
There are very straightforward differences, though. Like having a limited amount of abilities reduces UI clutter. This is generally true, and one can't argue, no way, having all abilities available is what reduces hotkey/ui bloat.
Limited abilities is more proactive, unlimited more reactive. Neither is inherently good or bad, right or wrong. I think a lot of us prefer limited, though, because the game becomes a bit more cerebral where you've got to plan ahead, and then becomes using the tools you have at your disposal to the best of your abilities.
Scenario: A pull goes a bit chaotic and an add is beating on the enchanter who is trying to mez the mob beating on the cleric. Your class has a mez spell. In an unlimited spell situation, you simply find the hotkey on your bar and mez the mob. Now, in a limited situation, you perhaps didn't load the mez because you've got an enchanter in the group with the job to mez mobs. You did however load your snare, because you're the only snarer in the group. So, you realize the dilemma and come up with a solution: you snare the mob, then blast it with a big DD, pulling agro from the enchanter and kite the mob long enough for the situation to get under control.
I don't see how the unlimited spell scenario adds more diversity to how a class is played. How can it when you're basically expected to hit the one-right-hotkey at the right time, the same as anyone else of that same class would be expected to do in the same situation. The skill becomes finding the hotkey you're expected to press among the 50 different hotkeys on your UI, which to me isn't nearly as interesting, fun or challenging as finding a creative solution based on the limited abilities you happen to have available at the time.