One of my early teachers in music production indicated that having a subwoofer in the recording studio will skew the true nature of the sound. We can all agree that without proper room treatment, even the best monitors won't be worth their weight, however, in a well balanced room with a nice pair of nearfield monitors, is a sub beneficial, or does it reduce the producer's ability to properly analyze the mix?
I think it all depends on what type of mixing your doing. In my opinion if its just pop music then its not really necessary (unless its hip hop). But if you're doing video games, film, trailers, or even television and people are listening to their subs at home then you might want to check out how much low end you are putting into the mix. You don't want to put too much and then it end up being too muffled.
So in some cases I think its necessary. I hope this helped.
You should definitely HAVE one, but have the option to turn it off...unless you're only planning on designing ring tones.
I do not use a sub when editing dialog, but I do for just about everything else. You're ulitmately mixing for full range systems, and you cannot competently mix and edit frequencies you cannot hear.