"Copy and paste is trivial no matter how you do it."
HA Ha ha ha... you were joking, right?
I think that X stands as a 20-year demonstration of the opposite proposition. I used to think like you did, and swore at one point a while back that I would clean up X's cut-copy-paste issues. I even publicly solicited help, although I found it curious that the more experienced that people were with it, the more they seemed to not want to get involved :-). Once I investigated the scope of the problem, I just gave up.
The current ICCCM solution to X cut-copy-paste is horrible: incomplete, inadequately honored, and confusing as hell for users and developers. This is *not* because smart people haven't thought hard about the problem, individually and collectively. It's because cut-copy-paste is hard.
I once put an entire software engineering class through a six-week exercise in just trying to provide an adequate specification for X cut-copy-paste behavior. It was a total failure, and the students mostly ended up hating me. They felt like idiots because they couldn't get their heads around this "trivial-looking" problem. I hope it taught them some valuable lessons about requirements specification, anyhow.
Cut-copy-paste is the UI monster in the closet. Back away slowly.