I may not have phrased it well, Graydon.
The advice that I have read, is that crabs need other crabs or apples near by, in order to produce fruit. They are not self-fertile, that is what it sounds like.
So if they need other crabs nearby, do the crabs need to be of a slightly different cultivar? If an orchard, say, was all propagated from one parent tree, then they would all be clones. Without another cultivar in close proximity, and available to the bees at the same time, would the orchard bear fruit?
How different do they need to be?
And your question brings up another one... if you can graft a branch of a slightly different cultivar onto a tree, would that grafted limb be more likely to bud and blossom in closer synch to the timing of the tree? Or do you have to pick a variety that blossoms at the same time as the tree that you are grafting to?
(hey, my study has always been horses, not horticulture.
Horses I have figured out... trees are still rather puzzling)