I commute once in a while (about 16 miles round trip), and I always use my singlespeed cross bike. The advantages I found with the single speed is less maintenance, and if you ride in the winter, you don't have a rear derailleur to freeze up with snow and ice. I enjoy riding singlespeed for commuting.
I agree with Walfredo - you need to find the gear ratio that works for you. You said that you don't make use of multiple gears since it is flat where you live - Is there one gear you always use? If there is one, then I would make sure you keep that gear ratio.