IMHO, for this form of multihoming, multihomers should *not* use
prefixes that that are part of the allocation of any of the ISPs over
which it multihomes. A multihomers address should come from
well-known managed PI blocks. Using part of an ISPs allocation
results in inefficient routing, unneccessary fragmentation of ISP
address blocks, and more complex policies for ISP peering.
What do we gain with having a dedicated PI block? Except administrative
overhead. In what way will routing be more efficient just because the
addresses are from a dedicated block?