This is largely because I run a server out of my house and I want to (a) have redunancy, and (b) balance traffic between the two connections.
Last night both lines went down at once! Twice! I was completely cut off from internet connectivity.
OK, not completely. I whipped out my cel phone and sshed into my ISP so that I could update the status page for my backgammon server. Using vi on a Palm is amusing.