It's highly unlikely that an apple tree (crab or otherwise) grown from seed will turn out to be the same as the variety it came from. This is all to do with cross pollination, and is why all commercially grown apple trees available to us are grafted using a known scion onto a rootstock of known performance.
Having said that, it all depends what you want the crab apples for, since the resulting tree(s) may turn out to be good (and unique) varieties themselves. Growing on a seed from an apple can be a rewarding experience, the problem is, it could be several years before you know whether you have a new classic variety (unlikely to be honest), or something totally unsuitable for your needs.
Mark |