There shouldn't be any reason not to make your frame as a button if you want to respond to clicks. The advantages of buttons are that they innately support clicking and you can set textures to illustrate the various button states (up, down, highlight, and disabled). You can make a frame do this, but it's a lot of work to get it to do so and honestly isn't worth the effort.
UI objects in WoW form a hierarchy in which more specialized objects inherit methods of the generic ones. For example, Buttons inherit everything a Frame has and adds more features while a CheckButton inherits from Buttons (and Frames indirectly) and adds its own stuff on top of that.
There's an illustration of this here:
Widget API