I think your point about the signal/noise ratio is telling. Thing is, though, the sort of signal one is seeking might characterize itself along different dimensions. For myself, for example, I usually start with a garment type -- and then start looking for instances that I think would look good on me. It's only after that that my filters consider price. So if I must visit multiple sites to find my candidate patterns, that's a challenge of it's own. And unless there's enough overall volume (e.g., travel) to make an aggregator site profitable, I have to do it myself.

Ravelry does have a price filter, so those who want to limit their search to free patterns can do so. I think it's for the best when there's room for everyone, with good filters to remove the stuff one isn't interested in.