I don't think I could ever justify spending my money on figures just to have a "complete" set! What's the point? There has to be some level of desire/yearning/interest there... I can't complete a line just so that I can say, "Hey! Look at me - I've got the complete set!"
I'm finding more and more, with Transformers figures, that as the years go by the quality seems to be diminishing in one aspect or another. I really REALLY love the Classics/Universe line... but I will happily pass on a figure if I think it looks crappy, or is a repaint of a figure from a previous line!
About the only TF line I ever intend to "complete" is G1... but even then, I won't go after any figures that I don't really want!