All you ever get from crab or crap apple trees is crap. Pretty flowers but sour fruit, mess under the tree, wasps in the fruit falls, and leaves to rake.
you can make crab apple jelly or you can use them to make pectin for other jellies/jams. (never done it myself but seen plenty that says you can do this)
if they are healthy why not use them to graft some regular apples to them so you get the tasty ones later on?
I've made crab apple jelly from a crappy crab apple tree. The jelly was good, took a lot of crappy little fruits, took a lot of sugar as they were very sour. I'd rather have a REAL apple tree, enjoy the blooms AND the fruit!