Fair point daz, but I think that to go down your route, which basically implies that every seat is pre-allocated, would entail a much longer boarding process than the 'new' Ryanair policy of only allocating seats on a selected number of seats.
Also, if I tried to book with Ryanair and then found that Mrs W and I could not sit together because only single seats were available, I might be tempted to switch to EZY, who state that they will try and seat pax together, regardless of any payment made.