Sure we do, hosts such as laptops routinely have different interfaces
connecting to different services with different properties. Today, we
typically don't use those at the same time, but it's not a big leap to
expect this will happen more in the future. For instance, if I'm walking
around a convention center with my laptop while I'm downloading a file
and I'm looking at a device over a telnet/SSH connection, I would like
both of these applications to use wireless LAN connectivity if
available. But when I'm out of range of the wireless LAN, I'd want the
terminal connection to switch to GPRS or something similar, but the
download should simply stop and resume later when there is more/cheaper
bandwidth.
And this is easily accomplished without any support from the routing
system today. All it requires is an ACL saying what sort of traffic is
allowed on a given link. In two words, access lists.