"So don't use it if you don't like it but why would deny its use to others?"
I am not denying use of anything to anyone. If someone wants to 'rate' some self-defined quality regarding individuals in their tree, there are easy ways to display their opinion. If they want to set up a tree-hosting website where only entries proven by original documentation are allowed, and employ myriad trained, experienced genealogists to monitor and vet the entries, that's certainly an available option.
But some sort of rating system really will not become part of the Ancestry.com overall MemberTree software any time soon. Most treebies will not sign on to it, and the idea that one cannot or should not do "research" by copying from trees (as implied by a low "quality" indicator, installed by ??whom?? on ??what basis??) is contrary to Ancestry.com's marketing themes.