I couldn't be bothered switching by season, if at all avoidable. There are enough sensible multigrades available out there, and OCIs can be long enough, that there's no reason to do it. I could conceivably put on so few summer miles I'd be throwing out perfectly good oil for winter usage. Besides, there are enough high HTHS oils out there with an appropriate winter number to take care of most needs.
If someone is insisting upon running conventional and isn't afraid of short intervals, then there is a slightly more compelling argument. When I was running 15w-40 in the old Audi in the summer, that worked, but some of the OCIs got a little shorter than I would have liked.