I guess I am in the "it depends" camp.
In my experience with visiting folks whose systems have a sub(s), 9 out of 10 have it set up incorrectly. I have often suggested they do as Phil has done and cross it over lower. To encourage them to live with it for a while and then, if needed, to gradually turn it higher. The BFD is a great tool but it still depends on personal preferences.
IMHO, most well engineered floor standing speakers do more than adequate without a sub. A sub may be required for monitors but this depends of a person's musical preferences, how loud they want to play them, and the size of the room they are in.
I purchased a Vandersteen 2W sub for my Magnepans and quite honesty have it turned down pretty low in it's current room environment. At my other home, I had it turned up higher to get it to be as intergrated as it is in my new home. So, room treatment or characterics play a key roll in whether one is needed or not.
A very long time ago a highly regarded and acclaimed audio engineer was conducting a clinic I attended and told the audience that subs are only good to use if they don't bring attention to themselves. They should also sound like a live performance (non-rock and jam band, of course).
From what I have read on many of the HT threads (at this site too) is the constant references of shaking the windows and waking up the neighbors five houses down the block. Does this really have meaning? Personally, I don't get this line of thinking.
Another great sub, for anyone considering options, are the ones designed by Audire. They are sealed units which incorporate the new XBL^2 technology. A 10" or 12" driver does wonders and are very easy on the ear.
Have a great weekend.