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 new routes. 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. Stephen