It's not just problems copying, its the fact they are much more prone to suddenly stopping working or developing errors for no real reason. A CD-RW may work perfectly, you check it, do your wiping and then come back to it and its developed errors whilst sitting in its case.
Their fairly flaky and the problems are inherent in the technology. I'll take something I trust over the re-use. Besides I've still got a big stack of unused CD-Rs as I rarely use them.
