You can still ride on the flats (aka tops). I routinely climb with my hands outside of the mitts. If it's for relatively short periods - like the icy spots you mention - there's no problem. It's the drops that really can't be used effectively once they're installed - at least not by me.

Your comment about getting zippered ones even though you don't need them for Shimano shifter cables is interesting. I routinely pull out the hands if they're getting a little too warm. It would be nice to be able to dial in the warmth level like with pit zips on a jacket. But I don't think the gap would be big enough or stay open enough to make any real difference.

But the bottom line: buy them. These things were a major leap forward in my winter commuting comfort. I can't imagine doing without them.