The reasons that I would recommend a MTB for a commuter are many: Older (early 90's), rigid (no suspension) MTBs can usually be found cheaply on your local CL. They are rugged and can handle commuting duties admirably well. New components to replace worn-out ones can be found easily and cheaply. MTB gearing is ideal for climbing any hills that you may come across on your commute. They are easily transformed into functional, all-weather commuters with the installation of fenders, racks, and lights. 26" tires are readily available for all sorts of riding; slick, "urban" treads for the city, knobbies for singletrack, and studded for winter commuting. You can even purchase a separate set of rims and tires for a quick swap for ugly conditions or if you want to hit the trail on the weekend. When you can afford the bike of your dreams the MTB can be kept as "Old Faithful" should your dream bike be down for maintenance. Lastly, if it gets stolen it can be replaced easily.

If you don't want a MTB then by all means find a road bike or hybrid. Almost any bike will make a great commuter when well-equipped.