"Crab apples" can be used in different ways. In this context, what they mean is that you will have an apple tree with unknown fruiting characteristics. Apples have a very wide variety of fruiting characteristics from tiny ornamental crabs that nothing eats to large eating apples and everything between. I'm growing lots of apple seedlings from seed. These are for deer and deer will enjoy a much wider range of apples than humans. I like the idea of having genetic diversity. Named apple varieties are all clones from a single tree that have been asexually propagated and grown for many years. They have stagnated genetically while disease and insects continue to evolve. Large stands of the identical tree become targets for disease.
So, I'm growing apples from seed, but I'm hedging my bets. I'm planting these seedlings in the field and grafting them above the first few branches. I'm using disease varieties of apples and crabapples with known fruiting characteristics. The first few branches will be from the native sexually propagated tree and the rest will be the known variety. When I get an apple from the lower branches whose characteristics I like, I will collect scions from those branches and use them to graft other trees. This way I get both a sure thing as well as the possible jack pot of a new apple with great characteristics.
So, the problem that you will end up with is a set of trees that could yield from nothing to something great and it will take you years to find out. If you graft those seedlings you can have the best of both worlds.
These two threads show what I'm doing with apples:
http://www.habitat-talk.com/index.php?threads/apple-planning-phase-transfered-from-qdma-forum.5536/
http://www.habitat-talk.com/index.php?threads/starting-apples-from-seed-indoors-how-to.6613/
Thanks,
Jack