You still have to do something for the functions with many local variables. In those cases you have little choice but to allow all N variables into the set, even if comparatively few are in any particular set at one time.
Sparse sets are also a great way to implement NFA state sets, where again you have a large number of possible set members but most sets are small, and you don't want to pay the O(all possible states) cost over and over.
Sparse sets are also a great way to implement NFA state sets, where again you have a large number of possible set members but most sets are small, and you don't want to pay the O(all possible states) cost over and over.