I think it really depends at what level of fitness you're at , and what you're trying to accomplish. Running will definitely help with overall fitness, and also weight control. However, at an elite level, you need to be sports specific, and time and effort spent on running is time and effort that can't be spent on cycling and recovery.
If you want to be the fastest competitive cyclist you can be, I think you shouldn't run, other than as some cross training in the off season. If you want to be a generally fit strong person, with strong bones, definitely mix in some running.
This off season I'm running for the first time, did a triathlon, and am trainging for a 15k. It's help keeping my weight down, but I feel like my on bike performance is down.
As the bike training gets into the prepreation phase, I'm going to stop running so that I have the strength to devote fully to the cycling drills and intervals.