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It depends a lot on what your wet weather/winter riding consists of.
If you go out and do one long weekend ride with not much starting and stopping and then clean everything then there probably isn't as much need to have 2 separate road bikes.
Commuting in a city means my rims have to be replaced every year on my commuter, and when the weather is bad I probably get through a set of pads in 6 weeks. Chains can go in a couple of months and when you get home at 7 in the dark and pouring rain then last thing you are thinking about is cleaning the bike. It gets chucked in the shed shed till morning when the process starts again. Clip on guards have rubbed away the paint finish on certain area's and depending on where I am working I can sometimes just take my bike into a secure area where it doesn't need chaining up but I have also had to leave it locked up outside in the east end of london without shelter.
No way on earth would I be riding my nice carbon framed team edition frame through winter even though its not dripping in dura ace!
For me, commuter and road bike aren't the same at all! 🙂
Although I don't clean either chain very often, and both last ages. In fact, the build up of black grime on uncleaned road chains seems to be the very best protection from the elements.
I do like a second bike, mainly for winter training. But this year I travelled with mine using it for trips to France, Belgium and the Pyrenees saving my good one from the clutches of baggage handlers as well as road dirt and salt.
My good kit is like new every time I race or just ride it. On a side note moving over to Campag I found it very weather resistant.