The actual full GearScore mod also polls a players statistics and will tell you how many (up to 5) kills they have on every other raid boss.
So it'll tell you if that specific character has, say, cleared Ulduar, ToGC, ICC, RS 5 times or more in 25man raid setting, and RS 4 tmes, or whatever they've done.
While they could be playing their partner/friends account, it doesn't say, sure.
That they're some top raid leader's love interest who gets consistantly carried is also true, but neither of these is likely, so it's a good indicator, better than them just having a high GS.
Of course, there can still be someone with 1k less GS and no raiding statists showing for that character who has done them on an alt and knows the fights, and can do it in a superior fashion to the first guy, this, also, is unlikely.
GS is good when you don't know anything else about a person. It's profiling, pure and simple. With nothing else to go on, you take the guy for the role with the higher GS, and you're that much more likely to suceed.
But yeah, It'd rock if there was some new mechanic, like, say, 'MVP' ratings.
Every time a raid finishes, everyone in the raid choses the MVP. That affects those players MVP scores in some algorithmic weighting of how many raiders voted for them (it'd have to be at least, say, 1/5th of the raid to carry any weight) and adds to their overall.
Then, you use that in place of GS, I'm all for it. :)