If they are selling room in the middle of the plane for a Economy+ type type deal then you can bet they are getting that room by screwing the other seat holders,
If the flight is short then its not that big a deal, but two or three hours in a cattle car is painful.
If I have to fly United then I just pay the extra fee, but I try to avoid that carrier altogether,
If it's just a few hours I can put up with pretty much anything. My recent trip was 7 hours from JFK to Amsterdam, and then 11 hours from there to Zambia. I did business class for those segments... makes a huge difference.