Maintenance issues aside, I'd still probably have separate summer and winter/offroad bikes. There's no reason I couldn't use my winter bike all year. I can put 23 mm tires on it if I really want to and even though it's a "beater" in some respects, it's not a bad looking bike. When it's shined up, alot of people would prefer it over my summer bike.

However, my preferred summer commute bike is one that's primarily about road performance with maybe a few token nods towards utility. It would be hard to find a single bike that didn't compromise either on summer speed or winter stability and traction.