I'd far rather have a specific seat allocation.
The passengers would rather have an allocated seat
The staff would rather allocate seats
So why not?? Easyjet claim that it's because it will slow down the turn-around, but I fail to see how. It saves all those silly ABCD announcements. Pax in rows 14-26 either board first, or use the rear steps, it's quite simple.
Ryanair don't because they operate a manual check-in system and it would take too long. But lets face it, if the facility is already there, how long does it take to say "window or aisle"?