I think you`ll find that they do a bit more than " open doors " mate.
The mere presence of a guard acts as a deterrent to would be knob heads on night time trains, as they`re the eyes and ears of the driver and railway old bill.
Lots of crime nasty crimes - assaults / robberies and sexual offences take place on trains / in and around train stations as they`re a universal magnet for wronguns.
Also time and time again, the guards act as a set of eyes for the driver when piss heads and others are trying to jump on the train last minute when it`s pulling off.
Simple question - would you rather your missus / daughter travelled on a night time train with or without a guard ?
* I know people who work on the railways, both on the trains and for the railway old bill. All of them to a man / women state that getting rid of the guards is a very very bad idea.