How can you tell if someone is experienced or skilled by whispers from global? You simply "trust" people you've never seen?
5k~5.2k GS is enough for ICC 10 Normal, provided the person has proper talents, proper gemming/enchants, and proper knowledge of his class and encounters. The thing is, it's not easy to find these kind of people on global (and again, how do you know if they're good or not?) so naturally the best option is to ask for overgeared toons. When you do that, higher the chances the people you invite are not too bad, because technically there is proof they have experience (their gear and achievements) and the overgeared toons will usually compensate their lack of skills and knowledge. Of course they will pull less DPS/Healing than they should (let's say you get a noob 5.8k Hunter), but it's the equivalent of a 5k GS guys that knows what to do and it'll (probably) work out well.
Basically, you don't know anyone from global and it's easier to trust and invite overgeared toons with achievements than "enough-geared" toons for your raid. And if it's relevant, don't you think it's just Molten or private servers - it happens on every patch of every expansion, at least from WoTLK onwards.