Well, First group seems to be VERY insistent on keeping costs low, regardless of the end result of that!
SWT for example, have it written into the current franchise agreement, that there is a requirement to have a Guard on ALL their public services, therefore they cannot suddenly change this until the Franchise runs out in 2017.
Safety of course IS the number one reason, as in an emergency situation, 2 members of staff (both Safety Critical) will always be safer than one to handle it.
Customer service is another reason, I think passengers like having someone present that they can talk to and direct questions to, especially during times of disruption.
The Guards that collect revenue also pay for themselves in a way, without them, Ticket Examiners would be needed, although these are less costly than a Guard, they are of course not safety critical staff and for the small difference in pay it is well worth keeping the Guard onboard!
Btw I'm not a Guard, just barrier staff, but I see many reasons against going DOO.