I have been following this relatively closely, partially because as somebody who commutes on SWR it affects me personally, and also because of my interest in railways. But I think one of the reasons the RMT is beginning to lose favour with passengers has been demonstrated in the past few posts. This whole argument over Driver Only Operation seems to have been constantly focused on by the media as a fight over 'the doors'. (Quite possibly because the acronym DOO is very similar to the word 'Door' - I only say that half in jest!)
There has been no explanation from the RMT - or, I suppose to be fair, one that's been picked up and published in the media - as to what the role of the guard actually is. I've started a conversation with a few people offline or on other forums/social media I frequent, and the answer to my asking what the guard does is generally along the lines of 'Opens and closes the doors, makes announcements, checks tickets.' And, a lot of the time, I've not really had a counter argument because actually I cannot think of what else a guard does, especially as on the SWR suburban lines, many guards only seem to do the first one, leaving the announcements to the automated system or even not bothering altogether, and doesn't work their way through the train even though sometimes they say they will.
(I am merely an enthusiast, I have no employment on the railway, and, yes, I realise I probably should have steered away from conversations about this subject without having a suitable rebuttal!)
Now, a lot of the responses above have shown what the guard does is often something that happens spur-of-the-moment and isn't exactly a 'duty' of the guard except under the banner 'keeping the train safe', and that is what my usual answer would be. But it would help justify their cause if there was actually something put out there that would explain what a guard does and why they should be kept. Maybe some 'case studies' or something.