313103 - There are guards on some Southern Metro routes (some in the Dorking & Guildford areas, etc.), although I agree that's the exception rather than the rule. The vast majority of other Southern services will have guards for at least parts of their journeys, except those services branded Gatwick Express. As alistair says, there are also FGW guards who work Turbos, and indeed there are also extra TTIs being recruited for some routes.
I think it is accepted that whilst safety is always a priority for staff, revenue work is usually seen as a reasonably large part of most guards' duties at many TOCs. This is particularly important where there are a large number of stations with no ticketing facilities (many rural FGW stations, for example) or where there are several different routes & TOCs, which may mean tickets have to be checked and excessed/replaced on board, if need be.
I agree and quite appreciate that on busy trains it is sometimes not possible to walk through effectively. However, unless a service is crush-loaded (e.g. the WLL and some other urban services), most guards do seem to at least stay in passenger areas. This is important in order for people to feel safe, so they can ask questions, so they can hand in lost property, so they can get advice on delays... you name it... and if the train can be dispatched from door panels, then so much the better. Even if their Avantix isn't working or something like that, a commercial guard can still be of assistance in the saloon areas, just by walking up and down & taking questions. Most guards understand this and I have to say that I rarely find them to be in the cab without a reason which I can work out. So I have no problem with the current system, but I do think that saloon-based duties very often need to include revenue work as appropriate, and most staff have no problem with this.