SWR want to have the flexibility to still be able to dispatch a train without a guard in the event of unexpected sudden circumstances meaning they are absent (e.g. other train delays, last minute illness), knowing a replacement can jump on at the next station or two down the line.
So many trains are cancelled or heavily delayed due to this issue in my experience, that I can see their point if the driver is sitting there waiting to go, and has the tools to do it. I eminently support it in fact. Why delay or cancel a train at Waterloo, and cause all the knock on effects, if they can get someone to jump on at Clapham Junction?
I guess the grey area is what these 'exceptional' circumstances are and where the line is drawn, and how long a train is run without a guard.
Ultimately at the end of the day this is still mostly about the RMT guards' powers to inflict maximum damage during strike action and their political clout. Sugarcoating it with safety, etc. isn't working anymore. Especially with the way their social media account seems to fly off the handle at anybody who dares oppose them.