This constant "weights are useless" theme that seems to have come about these board is starting to get on my pecs.
Weights are useless because leg strength has nothing to do with an endurance discipline like cycling. I have heard of a one legged time trialist. He put in similar times before and after loosing his leg. Because the limiting factor wasn't his leg strength it was his cardiovascular efficiency. By this I mean that the limit of his performance was how much Oxygen per second he could get into his body not how hard he could push the pedals
Weights are important to cycling because a key element of performance is balancing or stability on the bike. Steve Hogg in this article
http://www.cyclingnews.com/features/the-anatomy-of-bike-position claims that after aerodynamics and aerobic fitness this is the most important factor. So improving it will give performance gains. It is certainly the case that putting in big miles on a bike will lead to better core strength. However, certainly for beginners, a short cut to greater core strength is to do exercises, pilates or yoga.
To summarise, any weights that give better core strength are, in moderation, a good idea
Increasing leg strength isn't necessary and is a waste of time
As the specific aim being discussed here is endurance cycling, the most effective form of training will be ( uncontroversially ) endurance cycling. This form of training will probably be more useful than gym work