It's a great idea. I've been a single speed commuter year round for the last 4 so I'm biased.
Many good points have already been covered. True, it may not be as cheep as you think to convert, but it will be cheaper in the long run in time to maintain and parts. Proper gear ratio is important if you want it to be versatile. I prefer 42x16.
If you don't have a lot of hills and you don't mind your top end speed being limited by the gear ratio, a SS can do everything easily, simply, and elegantly. On top of that IMO you get a better work out riding SS, you cant hide behind gears.