Mainly punctures and wear. The idea is that in winter you a) arn't trying to ride as fast as you can, just training, and b) there's an awful lot of extra stuff on the ground that gives you punctures.
Note: the above post is an opinion and not fact. It might be a lie.
Don't think grip is great on any bike tyres. For me I go wider for a bit more comfort as speed isn't such an issue. I also think they'll help stop the p word from happening although really I should expect that anyway from my more expensive racing tyres!
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I go equally slowly in summer and winter. No punctures so far, touch wood.