I don't think many people here would claim that all full-on is bad. Rather, the reason why people use the term in a derogatory fashion is that a large portion (some might say the majority) of real cack that gets released in the name of psytrance is full-on. I don't know much about the history, but I think this is largely due to the Israeli market - the fact that psytrance has gone mainstream there has inevitably led to a lot of artists making music which doesn't deserve to be called "psychedelic" or "trance" but which is marketed under the psytrance label simply because the artists were inspired to make it after listening to the worst bits of Infected Mushroom and GMS's back catalogues. See Baba System for a fairly accurate parody.
Of course what qualifies as "real cack" is ultimately subjective, but the kind of people who hang around on psynews do have a lot in common when it comes to taste; there are people who love darkpsy and hate progressive, people who love goa and hate darkpsy and so on, but almost everyone here would agree that IM's The Gathering is good and Dali's Hectic is not.