Michael provided one reason why multihoming isn't always worth it.
The other reason is that pretty many problems in Internet are caused by
routing instabilities, and multihoming only provides more ways for
the instabilities to affect the "redundant" network.
Most of global routing problems can be classified as "Bysantine" in
theory of fault-tolerant systems. I.e. faults which cannot be handled
by replication of components.
In my experience, purchasing a UPS is a much smarter way to spend money
on improving reliability.
Note that regular telephone service does not generally employ multihoming
and dynamic adaptive routing, but has a lot better reliability. Something
to think about.
--vadim