I think NPCs present the best opportunity to experiment with suboptimal builds. They are expendable, you don't want most of them to overshadow the players anyway, and if you don't like them they can just disappear into irrelevance. But while they are still around, you can mess with expectations.
In particular, think of short races (-2 STR, smaller weapons) with strength-dependent martial classes. A halfling barbarian, for example. I would NOT want to play that in a campaign, but the fight against the PCs would be amusing. The players think he's going to throw rocks at them, but then he pulls out a greatclub and hits a lot harder than expected. Not as much punch as a half-orc would have, but I suppose that's useful for lower-level encounters.