Customer AS

PS There is a proof by existance that reliable operation w/o
  customer multihoming is possible. It is called POTS.

That's funny. Anyone who needs reliable telephone service
quickly learns how to multi-home between telephone providers,
inbound and outbound. AT&T periodically puts out a press release
saying their network is so reliable, no one needs to buy service
from anyone else. But people still do. If you want reasons
to buy service from someone other than AT&T, just ask your Sprint
or MCI salesperson.

Anyone who needs reliable electrical service, quickly learns
how to multi-home between electrical providers. The street
mains, a UPS, and generator. Some people even get a redundant
connection to another street mains.

Yes, it is complicated to multi-home between providers of
anything (telphone, electricity, Internet, bagels). A redundant
connection to the same provider might help in a few limited
circumstances. But generally, when a provider fails on one
connection, the probility their other connections are working
drop dramatically. When the electricity fails, using a different
connection to the same electical provider usually doesn't work
either. That's why people buy UPS's instead of second power supplies.