With this system it is near on impossible to allocate seats.
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)