My opinion (and thats all it is) is as follows...
Rain itself won't hurt the bike, but salt at this time of year will.
What speed is your summer bike ? That above all would dictate whether i use it in the winter or not.
I used a 9 speed very very occasionally in the winter, not wet rides, but i did use it while there was salt on the roads and maybe damp.
The next spring i was cleaning it and found that damp had settled in the front fork brake recesses, causing corrosion, causing the carbon to delaminate slightly from the steerer. Also i had furry corrosion on the alloy frame where there'd been any paint chips.
Now i always use an 8 speed steely for winter commuting, the chains heavier and more able to take corrosion and crap. Components are cheaper when you have to replace them. 10 speeds wear out quick enough in the summer, let alone when there's salt and muck all over.
My 8 speed is mudguarded up and i still have to clean it every two or three days, its unbelieveable how dirty they get. I wouldn't, couldn't subject a 10 speed to that...but thats just my choice.
Put it this way, after 18 months of summer riding only, ive just replaced the chain, cassette and chainrings on my 10 speed....cheap at just under £100. I think if you bring winter riding into the equation, you'd wear the transmission out twice as quick.