The reason that balance is important is related to the Original Position Fallacy. If you start a game of D&D blind; as a total newbie with no expectations other than "a cool adventure game", you are given the appearance of parity between the classes. Everyone expects that what they choose will be cool and powerful in different way, but that's not what was provided.

Imbalance can be fun if you know that going in; Rifts has characters that are literally as durable as tanks in the same party with "scholar of things the oppressive government doesn't want you to know". Similarly, in Mage: The Awakening, mages are explicitly more powerful and important than the Sleepers (the mundanes), but since you only play mages, there isn't that sense of grievous imbalance.

The difference is in both of those games, the imbalance was on purpose. Rifts is there to have a range of power levels, and Mage has the core concept of "magic is better than not-magic" and designed the game's lore around that. D&D's imbalance kinda just happened. While they wanted Wizards to be cool, they seriously failed consider how much power they were giving them, and greatly overestimated what fighter-types could do and kind of just said "Eh, they have feats, they'll be fine" without actually letting the feats do anythign interesting without also having a casting requirement.