This is a pretty well know urban legend that oringnated in the early '00s.
Supposedly it is true, but most modern Trac/Stability systems will still work when cc is activated.
http://www.snopes.com/autos/techno/wetroad.asp
I used the CC in my E53 occasionally, usually on empty stretches of road in rural Wisconsin and Indiana. I set it between 80-90 usually. Once, though , I did use it in pouring rain, at about 100-105 mph, in central IL (trying to outrun a tornado). I was fine, and didn't experience any loss of traction.
I have yet to use it in the E70.