Even if curses don't come up very often once you have them, you are going to hit your opponent with curses much less often too. So you'll get many more of them. What are you going to do with silver-copper-copper-curse-estate every(ish) turn?
If you buy a trasher early, it will take many more reshuffles to pair your deck down. Copper is horrible for engines, and not good for money either (just not as bad). Cheap alt-vp is really the only case where this is approaching reasonable, and even in many of those cases I'd rather have the tighter deck. Duke is the main case I think of, as it gives enough points to compete with province, but has a price of 5 - which copper helps you reach. Silk-road and gardens in most cases will be too slow unless you have a gainer like ironworks. BUT if you have ironworks, I'd rather have the tighter deck, at least for silk road. Maybe for gardens the larger deck size might be enough of a benefit.