I find that a lighter bike and more performance-oriented design changes the way I try to ride; it's more fun to ride a lightweight bike fast, especially when I'm wearing cycling shoes and using clipless pedals, which results in shorter trip times. I crank harder when my feet feel secure and when I don't feel a heavy bike resisting my movement. (For me, a significant weight difference isn't steel versus carbon, it's more like loaded-trunk, rack and pannier or not.) If that's fun for you, go for it, but if you'd rather ride casual, a couple of pounds won't make a big difference.