That's what I thought as well until I got my new notebook with a Matshita DVD-RAM UJ-840S drive. The drive can write CD-R at up to 24x but CD-RW only up to 10x.
Just for fun I tried writing a Verbatim Ultra-Speed CD-RW 16-24x at maximum speed (10x) on the Matshita drive, and the result was at least as good as when burning on either of my desktop drives at their rated speed. And I could also erase and re-write the CD-RW afterwards.
So I guess there are exceptions even to this rule.