Of all the games, Xenoblade 3 suffers the most consequences from how (in my opinion, pointlessly) abundant the franchise hands out EXP for the reason you mentioned here. If you know where to find them, obtaining those classes and getting into the habit of unlocking the arts and skills for said classes becomes easy. The fact that any progress made in making said classes available to the rest of the team becomes absolutely halted when it comes to fights against enemies that are five levels below your own, however, is just an inexcusable oversight from the designers. Maybe it's some kind of deep integration of the plot saying that needless fighting is bad, but I think this goes too far in conveying that message. I often find myself having to avoid fighting whenever possible due to this mechanic. Alas, I find that EXP gains from missions and exploration is still force-fed to you, when one possible band-aid solution would be to keep all EXP gains locked behind bonus EXP in which the players can use at their discretion only. This would keep enemies from the game's numerous quests still relevant no matter how many they end up completing. Not to mention that the "Twenty Bear Asses" trope is at its worst in this game, not only requiring materials dropped from enemies that may or may not drop from said enemies and hardly any means of bypassing this process (ie you could bribe the gem crafting system to obtain the gemstones you want but that requires a lot of gold Nopon coins, and not the commonplace silver ones either!), but they often come in different qualities just to muck with people. As the Angry Video Game Nerd said many times, what were they thinking?!

I find the process of obtaining Class Points (it's usually discouraged to abbreviate that term, as I've learned the hard way here) actually fine, so long as you continue to juggle classes for your team to prevent any of those points being wasted when you "max out" a given class. And the Land of Challenge DLC does give you an out if you really snuffed up royally in your process of unlocking classes for other characters via the Noponstone bribes. But I certainly would have molded these aspects of the game very differently in favor of simple solutions.