GS is like a University degree.
If you have a degree it doesn't necessarily mean you're smarter ( know how to play better).
However, if you're a business owner ( raid leader ) you have the choice of who to hire (invite).
Chances are someone with a degree ( high GS score ) will be better, but not always.
In general it works out because they don't need to be an Einstein ( some WoW hero ).
It's important to have some standards in order to maximize you rate of return ( drops/fun during the time you have to play ).
I.E. $100 000 profit per $50 000 salary, or 6 bosses per 3 hours spent raiding.
It's human nature and the benefit of being a boss (leader).
If you lead you'll get your way. Especially in WoW.
If there wasn't GS there would be something else.