I read/watched I cannot remember which now as it was two days ago that Mark Cavendish started using weights in the gym to help improve his sprinting ability as others had caught him up on the aerodynamic front. Not sure if this was just leg training but he did say that gym/weights did help.
Just from my experience on the MTB, I am pretty crap when it comes to climbing as my mates can usually overtake me, but when there are short steep climbs I can power past them. The trouble is they will always catch me up. So in the end it makes sense that for endurance if you can maintain some sort of speed on the flat and also climb then that is all you need.
I train weights in the gym because I have done so for over 20 years, but if I wanted to be a serious endurance rider and a coach said drop the weight training I would. But I am not an endurance rider, I'm not even a very good rider and I like training with weights.
Going off topic slightly, I used to train in an Amateur boxing gym and the coach used to go bonkers if he caught the smaller guys using the weights as he would say that it slow them down, not sure if this is still the case, but modern day methods do involve more full body workouts eg TRX etc. I remember seeing a 9 stone fighter sparing spar with a real muscled 12 stone with less experience, the lighter boxer was quicker, stronger and was hitting harder, and all he did was hit the bag, skip and ultimately spared three times a week!