Ultralight fly rods making fishing for, and catching bluegill, small stream trout, and such, MORE FUN. That's the name of the game here right? We fish because fishing is fun!
So why would I want to use a 5 or 6wt rod for bluegills, that's not so much fun.
I use 2 and 3wt rods for about 95% of my fishing.

J.