Why in-game combat can challenge the players' tactical skill, but social encounters must never challenge the players' social skill, is an attitude I just can't understand.
A player with a PC in combat may suck at tactics, but he can ALWAYS fall back on the roll of the die & the combat rules to help bail him out*. A player in a RPG without a system for simulating social skills has no such safety net.
I mean, I know jack all about the practice of martial arts beyond what media (movies, documentaries, books, comics, etc.) and a single day in a karate class have taught me, but I can still play a really cool martial artist in most game systems. Were combat handled the way some treat social skills, I'd be limited to playing brainy wallflowers with artistic talent.
* unless there's some RPG out there that bases combat results entirely upon what players actually know about combat.