A Couple of Points
Having used the LCCs extensively over the last 5 years I have a couple of points to make:
1/ One of the reasons that LCCs are low cost is because of high utilisation of the aircraft. Therefore high punctuality/regularity is essential. No allocated seating makes sure that all pax are all at the gate and ready to get on to the a/c as soon as possible. I have often found that the a/c is full/ready to go a good ten minutes before departure time. So EZY must be looking to trial whether the added income from allocated seating is more than the cost of delays?
2/ One of the main benefits for me and my wife when boarding a LCC without allocated boarding is that we can see where the screaming babies and stag/hen pax are seated and can usually avoid them if you time your boarding well. Works out most times but the trick is to time your boarding.