I'd definitely prefer a system where spells weren't just outright replaced by a better version at Level Up, but learning new spells with better parameters can still be a good addition.
Whether they allow us to strengthen individual spells by training them in some way, or just use a simple INT mod, the spells really need to have some diversity, and preferably some customization, too.
For example, the only difference between Fire and Water are their element in the game (and I think their strength and cost differs slightly). Fire acts the same way as Water-- it gets to the mob in the same way, hits in the same way, and has the same effect (damage) to the same area. This leaves little thought for which spell to use-- the choice is generally extremely obvious. If instead spells were more varied in these ways, there would be more tactical consideration.
For example, Thunder might be relatively inaccurate, and have a chance of misfiring on the caster. Summoning a blob of Fire out of thin air would predictably do a lot more damage than summoning a blob of Water against most enemies, unless maybe that spell wasn't Water, but a Drown spell that entered the targets lungs. Water might instead cost less than fire, cast more quickly, and have other effects besides damage (like turning the ground beneath the monster into mud or ice). Generally within element there would be spells with a wide range of properties, so that all spells would have more than a couple of differences. i.e., the difference between Fire and Water isn't, "Well, this one is fire, and... this one is water."
And what would be great is if these properties could be customized, so that my Aeroga might be one incredibly powerful burst of wind that hits for massive damage, while someone else's created a huge AOE blowback that sent enemies flying away. Or better yet, this ends up being a macro customization so that that's my /default/ Aeroga, but I can choose to change the way the spell will manifest midbattle. e.g., I'll get to choose to some extent what the MP cost, cast time, area of effect, and attack effects will be for each spell I cast.
Ok, now we're going to get slash fiction of Wint x Kachi somehere... rule 34 and all...
Never confuse your inference as the listener for an implication of the speaker.
Good games are subjective like good food is subjective. You're not going to seriously tell me that there's not a psychological basis for why pizza is great and lutefisk is revolting. The thing about subjectivity is that, as subjects go, humans actually have a great deal in common.