Seems to me (I don't have my reference books handy here just now, sorry) but I thought the average ear worked between 20hz and 20khz, maybe a taste lower at each end, but there ya go. So if it can be heard, and it can be recorded, then shouldn't we at least try to reproduce it? And to do that, we need speakers that can get there. For the most part, the upper range is no problem, many manufacturers build tweeters they claim can operate to 20kHz (ProAc claims 30 kHz on several of their models). The lower end is tougher, operating ranges for "full range" models vary from the upper 40s hz to as low as the aforementioned 20hz, for monitors the low end is generally in the 30s, but can be as high as 55hz (Your mileage may vary). Subs can therefore fill a roughly 10+ hz "hole" in the audible spectrum, just based on operating ranges (and accepting the premise that most subs will go down to, if not below 20hz). This is a huge simplification of the issue, I know, but enough people have written books about this stuff, and Audiogon isn't a publishing house. My point is, I think, still valid - to fully exploit our native abilities we need gear that will present the full audio spectrum. A sub helps many speakers achieve that. Naturally, like all gear, how it's set up will effect the sound: a sub that "flap(s) my clothes" may be appropriate for HT (I've been next to a 122mm howitzer firing at max range, not only do your clothes flap, your brain rattles!) but at normal, non-bleeding-at-the-ear levels, listening to typical music, you shouldn't even be able to tell where the sub is located in the room. But in my opinion and experience, you will be able to tell whether there is a sub in play or not. Whether you will like what it does or not is, like all things audio, your preference.
I can't say what the best sub-sat setup is that I've ever heard (I've heard many in many listening sessions at many places and times), so I'll cheat a little and just say I like both Hsu and Velodyne subs, but own a Paradigm 'cuz I bought it with my first system years ago, and am too cheap to replace it just yet!
chas