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.
The actual boarding is very marginally quicker with unallocated seating but like you say it is the fact that many passengers like to be at the gate in good time is what actually speeds the process up. That said there are still certain types of routes where passengers are late to the gate on a regular basis and consequently get offloaded. Many of Easyjet's routes don't carry that much hold baggage and so offload is not a problem whatsoever in many cases. IMO the gate should close 5 minutes earlier in order to accomodate allocated seating.
However, Easyjet will not accept any real damage to it's OTP which is why this is strictly a trial
I'm of the opinion that allocated seating is the way forward and with a pro-active crew onboard, seating passengers shouldn't be an issue.