You may be able to get away without powering it separately. I was reading the description of the Monoprice in-wall sub and it says "Pair these with each of your main speakers for clearer, more well defined audio and wider sound staging. Bi-amp them with a capable amp or receiver to get the ultimate audio experience." That would imply that you can run it full range with your fronts, but would sound way better if it had a separate amplifier.
They specifically say that In-Wall subs are not intended to replace the LFE subwoofer. If that was your intention, then I would not go that route. If you purchase a decent powered sub and set your speakers to SMALL on the receiver, you should be good to go. I am running Phoenix Gold In-Wall speakers for the L-R speakers with an old Polk center channel. I have a Hsu STF-2 for the subwoofer and everything sounds pretty good.
Many people say that in-walls are bad, unless you spend a fortune on them, but I have no complaints. Mine are an outside 6" fully insulated wall (I removed all but about 1-2" of insulation behind the speakers). Maybe coming from more of a car audio background where the frequency response is less than optimal gives me a poor view on how it should sound. One thing I noticed running test CDs through it and listening to music, my speakers produce a very nice phantom center in 2.1 mode. The first couple times, I went to the center channel to make sure it was off.
They are a good way to save money and more importantly space. I would stay away from the in-wall subs unless you plan to combine them with a true powered subwoofer. I have been tempted to add one to the center channel since the midranges are only 4" (I think). In that case it should be beneficial. My fronts are 8", so I opted to run the receiver SMALL and stick with only those without an in-wall sub to drop the bottom end down.