I'm going through a similar thought process, being just about to replace my 10 year old Saab 9-3. I aim to keep the new car for ~ 10yrs too. Do I trade in the 9-3 and drive the new one year round, or keep the 9-3 for winter use.
I think I'll trade it, based on:
- hassle of insuring a 3rd car / switching insurance
- likely repair costs on an old Saab
- space to store a 3rd car
- the Saab was never rust proofed and has no visible rust anywhere after 10 Ontario winters. Interior has also survived salt / mud etc with no stains
- joy & safety of driving a new car, especially in bad conditions (it'll be AWD, plus all the extra gizmos you now get)
- handy to have the hatchback space / don't care about the interior for carting stuff around ... my Saab pick-up truck

In the "pro-Winter beater column, I can really only think of
- trade in value is only approx 3 x monthly payment on the new car
- plus the obvious "keep the new car in good condition"
Cheers....