Not really being ripped off... Every single one of the 108 normal leg room seats has to be allocated to someone before anyone [except United frequent flyer program members] gets the extra leg room for free. And almost invariably if the flight is NOT full, what happens is that 108 passengers who haven't paid for extra leg-room are crammed in the back, whilst those who have paid/qualified for extra leg-room not only get the extra leg-room but also get the empty seats next to / between them. It often makes for a significantly more comfortable flight than being in the normal leg-room seats. Despite there being no cabin divider inbetween normal economy and "economy plus", in order to protect the benefit for those who have paid for "economy plus", United flight attendants are getting pretty good stopping passengers who haven't paid the add-on from creeping forward into extra leg-room seats after the doors are closed.
Its the same on most other airlines actually - in the end, if no one pays for extra leg-room (easyJet, bmiBaby, British Airways) those seats will still be sold as normal seats and someone might get lucky and get the extra room for free when they check-in... Passengers who want extra leg-room are paying for a guarantee they'll get it, rather than taking what on many occasions would be a rather significant risk of ending up with a normal seat.
Andy