The biggest problem is the best position for main speakers is often the worst position for bass. You may not be getting the bass your speakers are capable of when in the best location for other sonic aspects. Subs allow you to position your speakers in their best position and put the subs where they work best. Using the small sub you are considering in the correct location will probably make an instantly noticeable improvement in your reproduction.
Plus if your subs have a high pass function(not all do) they will significantly reduce the load on your main speakers and improve the upper bass and lower mid-range.