Bikepacker and aikigreg have it precisley correct in regard to strength training. The site that aikigreg posted contains the book Strength Training Anatomy
An absolutely superb read for specific muscle training. There is also one for women. Amazon sold and shipped both for near $15US.
I have trained cycling/Tri clients for 12 years now. I will never leave strength training out of their programs for the exact reasons Bkpkr67 posted. Father Time, k/cal expenditure, increase in power, and reduction in overall fatigue overtime in the saddle.
Yes, the pros strength train. They may not bodybuild. It is not necessary for us to BB either. They may keep it moderate; however, the ones that do ST understand how to keep their job longer, than ones that don't.