I can't really see why, as long as the provider has punched the
appropriate hole for your aggregate in their filters. More specific
routes always win out. Or am I missing your point?
The point, I think, is the effort involved in using global route
announcements to solve your traffic engineering problems.
When you use provider-assigned space, you have to coordinate your
intent to add entries to the global routing table with the provider
who assigned the space and the providers that you want to accept the
When you use provider-independent space, you get to decide to add
entries to the global routing table pretty much all by yourself,
modulo running afoul of the occasional provider that does not, by
default, buy into solving local traffic engineering problems in other
people's networks using global routing table entries.