I've noticed that, too. I think it has to do with "comfort level." That is, when a mediocre model gets posted, many members will compliment it to no end, because it gives those people a certain comfort level...because they know that they, too, can build a model to that level. So in effect, by praising a less than praiseworthy model, they're actually validating their own skill level and in a backhanded sort of way, complimenting themselves. Sort of like saying, "hey, cool model, looks fantastic... and I can do that too!"
I'm no Sigmund Freud, but that's my take on it.