Totally depends on the game. Some games, like Ratchet and Clank Size Matters, yes for the final boss. Games like Brok The InvestiGator, no because I found the combat easy on the hardest difficulty.

I have the opposite issue, so yes. I don't particularly enjoy having to constantly pay attention to every enemy, but I enjoy learning a boss fight for an hour or two. I've also played a few games where dealing with random enemies felt harder than dealing with bosses due to sheer numbers, and it would help with that too.

So I don't think it's really a design problem. If you know exactly what you want your game's experience to be, then don't add it. But I'd argue for most games it isn't integral to the experience how the difficulty of normal mobs vs bosses compares, and people have different preferences for it.

I think, a lot of games adjust the difficulty when you loose too many times. Don't ask how I know...

No that makes no sense lol