We have been filtering subnets of As (except 39) Bs and Cs for 5 months
-- but outbound, not inbound (ie nobody behind us can announce these
things except by special arrangement; we still hear other people's
funny stuff). One problem we see is European offices/branches of US
companies who want to use parts of their "company B" for a European
connection, instead of using local provider space; filtering subnets is
a good incentive for them to overcome that desire.
It's clear that the Net needs fewer announcements and this is one way
to go; and by filtering essentially "rogue" networks inbound (towards
presumably interesting things), one likewise provides an incentive for
better aggregation. However, I think Sprint's approach is rather
heavy-handed and probably a toad counter-productive in the long run. I
would consider it reasonable for Sprint to filter their own funny stuff
(like we have done) for a considerable length of time before filtering
other people's funny stuff; if for no other reason than proof of
concept on home ground. Plus stones and glasshouses, etc.