From what I've read on here, the microphones aren't generic and you should try to get the Denon mic. If it were me and I couldn't find one for cheap, I'd probably use a PC mic or the like and see how well I liked the results.
The auto calibration sets speaker levels and distances, and it applies some equalization to try and get a flatter frequency response (to adjust for impacts due to your system and your room). It is valuable for 2.1, too. Some people here love it (myself included), while others don't. I found it to make a very noticeable and positive improvement on how my NHTs sounded in my room. I have a Pioneer that uses MCAAC, which is different than what your Denon uses.