Before computer check in, there was a very fast way of allocating seats. The check in agent had a sheet with a graphic of the aircraft and each seat was represented by a sticky label, e.g. 5B. When s/he allocated that seat to a pax, the sticky label was peeled off and affixed to the boarding pass. Much faster than computerised check in these days (although not so thorough)
Maybe in the days of operating a 30 seater turboprop, but if you're checking in a Ryanair 738 with 189 seats you won't get away with one agent. This means the staff have to pass the sticky seat-plan back and forth amongst themselves which is hardly ideal.
The check-in system that I'm using is very quick. You don't even have to enter a seat number. You can simply hit W for window or A for aisle and it will seat the passengers to maintain an even bay split. If you have somebody who is particularly fussy about the row they want, then of course you can enter the exact seat. It's no extra hassle really!