Agreed.
I've been carrying multiple cameras (and sometimes two SLR bodies) for awhile now.
One of the main reasons to carry 'multiples' is this "what if something breaks?" factor. Depending on what you're doing, you might be hours or days away from someplace where you can get the gear repaired (or replaced). As such, failing to have a backup means lost time, probably lost money, etc, for whatever you're doing.
Depending on your needs, budget, etc, I do want to point out that a P&S can be a viable backup alternative to an SLR. Even if you want to go into the "its not as good as an SLR" debate, clearly, a P&S is clearly far better than having nothing...it can be cheap insurance, even if it isn't pendantically as capable. Besides, its nice to have a pocket camera for when you don't want to haul around the bigma.
For SLRs, I only have one digital body, so my 'backup' is one of my older film bodies. Nevertheless, there are advantages to this in that I would typically have one camera set up with each a different focal length lens. This makes swaps fast ... and dust free.
Which one is more important varies on your application: the trip to Africa last summer made dust the overriding factor, whereas my niece's wedding this summer made fast swaps important.
Plus since the film isn't a crop body, I still have a 20mm WA capability, although I do need to get that film digitized during developing. Its a reasonably fair trade-off to live with until the price of the Canon 5D (full frame dSLR) comes down to my price range.
-hh