What does "limiting your imagination" have to do with restrictions?
It has to do with the fact that I can "imagine" a character that simply can't exist due to mechanical restrictions. Not roleplaying, not story, not campaign restrictions. Just the rules written in the books.
This is something that I have heard several times but doesn't really make sense. Let's take the Bladesinger for example. This is a class that is the epitome of elven culture. It is a class that is considered to be the highest honor in elven society. It is a secretive and guarded class that elves risk their lives to keep the techniques a secret. Now I know there could be reasons as to why but niche ideas are not the reason to open it up to everyone. So in every campaign that you play in the elves made an exception to the rule to allow you to learn the art of Bladesinging?
The problem is that you've now assumed something about bladesinging. You've given it a place and a meaning. You think this makes something special . . . I think that this makes base assumptions about everyone's campaign that don't need to be there. But that's a different thread. My question is, what IS bladesinging? The art of blending music, blades and arcane magic into one. Is there some sort of genetic restriction that means that only an elf can do this? Or do the rules of the world exist in such a way that anybody trained to use the abilities can use them? But no matter, you selected a very extreme, specialized example. I can see a world where bladesingers are only elves . . . it won't chafe. But you're tying the same logic in to try and limit BASE classes, core material, the fundamental building blocks of the character design system.
To me, the ultimate example of limits for limits sake are class skill lists. Since 2e, you're severely restricted in your ability to take skills outside narrow lists of what someone thought would be appropriate for your class. Right now I'm playing a psychic warrior who grew up as an animal handler. But I don't have handle animal as a skill. Why? Because someone thought that the class shouldn't have that skill on their list, and because being able to take that skill would be SO overpowering that there should be no mechanism for adding a class skill to your list. It would be game breaking, I guess?
I think it's better to have restrictions that can be hand waved than vice versa.
And I think the opposite. It's easier to take away someone's options than it is to add them in. Because the game rules and balance should work assuming every option.
Paladins smite evil. Remove the LG restriction and what do they smite? Still evil? Or do we now have to change a rule, make up something new.
Paladins smite opposition? Install an LG restriction and . . . they still smite opposing characters. Same rules still work.