Andy_K 

I'm not convinced about the extra maintenance argument. In general, maintenance is a function of mileage. Assuming your total mileage remains the same, the total maintenance should also be about the same. A bike that never gets ridden needs very little maintenance (although I admit it does need some). I think there is even a case to be made that you have less maintenance to do with multiple bikes.

There's a theory that if you have two pairs of identical shoes and wear them on alternating days they will last significantly longer than if you wore one pair every day until they were worn out and then wore the other pair every day until they wore out. The theory is that the "rest day" allows the materials to relax or decompress in some way. Apparently this theory has been tested with shoes and shown to have some correspondence to reality. I have a hunch (though I've never attempted to test it) that something similar might happen with bike tires and perhaps other components as well.

At the very least, if you have one bike that gets ridden in the rain and another that doesn't, the components together will see less total wear than if you had just one bike that you rode rain or shine. My reasoning is that if you have just one bike, the drivetrain will accumulate extra grit on the rainy days and unless you do an excessive amount of cleaning some of that grit will still be there when you ride it on dry days, so the dry days put more wear on the components than they would on a bike that was only used in fair weather. Also, those who live in places where it doesn't rain every day nine months of the year (obviously I'm just theorizing here, as I can't apply personal experience) are likely to ride the rain bike fewer times before cleaning the drivetrain thoroughly after rainy rides than they would if it were their everyday bike.

On the other hand, maybe I'm just an addict trying to justify his habit.