What is all this talk about Burr knots about? I have some on a few trees in low areas, not a big deal. To answer your question Bows, it could be a weak area (down by the graft union) where different problems could effect the trees. Breakage in high wind, pests/ pathogens like fire blight, borers, woolly apple aphids etc. Normally the only trees effected are in moist areas. This is one reason why I have always suggested using window screen as vol guards to get better air flow at the base of tree. Also a good reason for good ground maintenance, keeping tall weeds down.
I wouldn't loose any sleep over burr knots. Screen them good, keep the weeds down and the more sun, the better. If you are using a good spray schedule on the trees, all the better.
I've got some really nice trees on Malling, Buds and Genevas, I don't go crazy about worrying about them except to match the root to the size, growing system and disease resistance. My conditions here are optimum for apples but some of you have things like extreme cold and poor soils, so I can understand trying to match the proper roots for those conditions, but burr knots are lower on the list when selecting a root, unless it's abnormally humid where you are intending to grow them imo.