This isn't so much an issue of warm-fuzzy technical fairness, more than
it is one of provider (interior) network stability, and when it comes
to the latter, the providers who are filtering on prefix length are
doing so because they feel that it is in their best interest. I would
suggest that the largest percentage of flapping prefixes in the global
routing system belong to prefixes longer than /19.
This is not to say that they could be economical incentivized to
accept routes for arbitrarily long prefixes.