Sometimes more-specifics are needed to compete against more-specifics being
incorrectly advertised by other people. Sometimes folks _insist_ on cutouts,
and you have to go route-to-route against them if you want to keep your block
intact. I hate this, it's bad business, I don't do it myself, I don't let my
customers do it, but I've seen it often enough to have it be worth mentioning.
If you've got a larger block (say, a /16) and a customer with a /24 becomes
dual-homed and wants to split the incoming traffic on his links, you're
stuck advertising the /24 as well as the /16; otherwise, their 2nd provider
will always win for incoming traffic from most of the 'net... However,
the 2nd path to a route consumes a LOT less memory than an additional
path.
We do this for 1 customer right now, and possibly 1 in the near future.
Luckily, the demand for this is mostly limited to ISPs rather than
other types of companies...
Avi