Whether a sub works well with a speaker depends on many things - mainly the quality of the sub itself, and how it is set up. All being well, you should be able to close your eyes and not be able to tell that you're listening to a small pair of speakers and a subwoofer. If you can't, the sub is attracting attention to its or either because of incorrect setup, or producing unwanted noise in the form of distortion or cabinet coloration.
If you're going to do it, do it properly and don't skimp on the sub, otherwise you're better off without.