+1 on all the "they give better coverage" answers. I have one bike which is good for all situations - it has a rack, with panniers permanently mounted, a saddlebag, wide-enough tyres, permanently mounted fenders, light mounts - there's no commuting situation I won't use it in. If the weather is good, or the route is not too potholed, or I don't need to leave it locked outside, then I might take my track bike, or my vintage roadie, or my chrome single speed, but no matter what, old faithful is there and ready to go, no thought necessary. 1 bike is never enough.