I am not sure that having pre-allocated seats other than for extra leg-room seats is a great idea. In all the years we have traveled with EZY Mrs Wowzz and I have never sat apart, and we have never purchased Speedy Boarding. We have also managed never to have to sit near fractious children, as they have already been seated when we boarded.
I would also have thought that seat allocation would have significantly slowed down the entire boardng process, unless a strict control was enforced with regard to boarding by seat roe numbers [ie those at the back board first]
The main benefit will be to stop those couples [you kow who you are!] that sit at an aisle and a window seat, challenging anyone to take the middle seat! If you want three seats, PAY for three seats!