My focus is endurance cycling - randonneuring. Up to 20 hours on the bike, sometimes several days in a row. My opinion is that while my primary power may be best developed on the bike, there are other muscle groups that benefit from strength training. Neck, core, hands, at least. While cycling uses those muscles, I don't think it works them to most effectively develop all day strength. I can only do so many 20 hour rides a year, and I don't think short and even intense rides strengthen those non-primary muscles as effectively as resistance training would.
For pure leg strength, specifically for short bursts of power like steep climbs, I tend to think resistance training is worthwhile. Ideally that might be possible on the road, but the realities of the road in most place means traffic, debris, obstacles, navigation, etc, whereas in the gym you can focus purely on the strengthening activity. I've had many a hill repeat session interrupted by a cluster of cars, but never a set of leg extensions. Clearly if you're a cyclist it's necessary to learn how to deal with those road situations while delivering power, but sometimes they are not conducive to developing power.
However, IMO saddle time cannot be replaced.