Huh. I disagree with everything about that. Going from friends to lovers, in my mind, is one of the most wonderful things that can happen. You have a solid foundation of trust and respect to build upon. You've been through a lot and pretty much know what to expect from the other person. I don't see how Spike and Buffy are better. Going from hating each other to firey, loveless (on Buffy's side) sex, to maybe mutual love? Seems like a rocky road to me. It's built on what, former disgust and repulsion? True, Spike is a changed man, but most of the time, a change that drastic doesn't happen.
Having said all that, I know that not everyone likes the friendship-->love thing. My sister, for instance, has said that for her there's a clear line between the two, and has never had a boyfriend who was a friend first. Different strokes for different folks.
But I don't really think you can call Cordy a rebound the way Riley was. Riley happened right after Angel left. Cordy and Angel have been working together for four years. If Angel was a human without a curse to worry about, he would have been able to move on and have other relationships. That's just life. It doesn't mean he loved Buffy less, it's just that time is the ultimate healer. Under normal circumstances, he would have moved on.