"Worthless" is far overstating the case: in general it is hard to prove very much about the true motivations of philosophers, particularly long-dead ones. For example, it is quite likely that the exact lines of reasoning in Descartes' Meditations was contrived to reach the conclusions he wanted to reach beforehand, but to say that makes the whole thing "worthless" is pretty silly. Many philosophers can be criticized as developing rational arguments for positions they hold intuitively.
