I've heard/read olympic weightlifting trainers Glenn Pendlay and John Broz and various olympic weightlifting athletes -- Clarence Kennedy for example -- say (and I will try to cite these when I get a moment) that training anything other than the recruitment patterns utilized while performing the movement is a waste. And that makes sense. Becoming exceptionally good at something means practicing it, not practicing other things. You don't see oly lifters doing bench press, because nowhere in the movement they wish to master is a bench press included.
The same mentality should be adopted for any movement pattern. If you wish to be exceptionally good at throwing trash bags in the back of a garbage truck, be a garbage man, not a swimmer.
Now, say a portion of the movement is identified as a weakness. Continuing our oly lifting example, say (to simplify things greatly) the lifter is able to jerk more weight than he can clean, a partial movement pattern may be practiced to train the lifter on strengthening his or her weakness (in this case, the lifter would work more on the clean portion of the movement). The same mentality could be adopted for improving on weaknesses in any portion of the movement. In the case of pedal stroke, this would likely require some analysis on a dynamometer or from a professional.
This boils down to your goals. If you wish to be a competitive cyclist, losing muscle mass in every part of your body other than what is needed for pedaling is beneficial and desirable. That's the extreme end of the spectrum. If you wish to also play football on weekends, train other stuff.
In regards to injury potential, that's completely subjective. Are you worried about injuring yourself walking down the street, or trying to flip an 800lb tractor tire? If it's the latter, then, yes, your leg muscles may be so powerful that they can pull your spinal erectors if you have not practiced the muscle recruitment pattern of flipping a tire. The same is true for most things. Try sprinting at full bore after having only done body building movements for a whole year.
Should weightlifting be used to strengthen underutilized muscles? I think this is a poor question. How do you define underutilized? Do you mean underutilized in cycling? Do you wish to improve on your cycling? If you want to get good at weightlifting, lift weights. If you want to get good at pedaling, pedal. If you want to get good at pedaling on 100km courses that are entirely uphill, pedal on 100k uphill courses. Progressive overload is the essence of all training.
