If your goal is to achieve the 'ultimate core' - then crack on. If, however, your goal is to get good on a bike, then simply riding a bike will give all the muscles involved in riding a bike the perfect workout.
Generally, I'd agree with that but (as I just posted in another thread) my core/back never seem to get fit from cycling as fast as my legs/heart/lungs do.
I am riding 6,500+ kms a year with 100,000+ metres of climbing, That is enough to give me reasonable cycling fitness but I still get backache on tough local rides (100+ kms, 2,000+ metres of climbing).
Lack of flexibility might be a factor too - if I tried to bend over and touch my toes I would be lucky to get within 15 cms of them.
The problem is that whereas I enjoy riding my bikes, I find off-bike exercise a bit boring so I never do much apart from the odd hilly walk.