Mechanically, it's valuable to have all stats have value both in and out of combat. Otherwise the combat value of characters starts to overshadow the roleplaying value and the game devolves to a pure combat-fest. If a stat has no value in combat, then it turns into a dump stat and you may as well not have people pay for it at all. It's rare, I think, to run into someone who's willing to be ineffective in combat in exchange for being the party face.

D&D 4e expanded on this trend by having multiple classes of varying types that had each of the six character statistics as the primary statistic, so it's possible to create a character with one, two, or three strengths in almost any combination and have at least one or two classes that cater to that combination.

As far as I know, many other RPGs don't have this problem because they aren't oriented around killing monsters the same way that D&D is, but other systems will still have forms of spellcasting based on force of personality because it's flavorful.