OTOH it is entirely possible on a slackly timed 2 car DMU on a branch line...
Just getting the ramp successfully out of the cupboard on a class 170 can be a several minute task in itself
I always find the irony of such statements being that with the exception of a genuine shuttle like Romford to Upminster the guard will provide the most value on such services.
On suburban trains the stops are such that they're usually demoted to a mobile train dispatcher, dealing with the odd emergency or incident (see Cross City, Birmingham, where the guards are effectively chained to a cab on a 6 car 323). I find this less of an issue on our Worksop services, as the stops are 3-5 minutes apart rather than 90 seconds and I can walk up and down a 170 operating the doors in each coach in turn all day long if I need to. I'll easily take 400 quid in revenue on a round trip, all whilst operating the doors in accordance with our rules. If they were much closer together I'd struggle.
On IC trains you tend to have a full train crew and station staffing.
On rural services you tend to find there are no staff anywhere but on board the train, so you pay for a driver and guard. They inevitably get involved in the full range of nonsense from chasing cows to chopping up trees to the odd fatality, on top of selling tickets, dealing with customers and reporting issues along the way. When something does go wrong, response times are usual crap - I have rarely waited less than an hour for external assistance when stranded, quite often much more than that. I once hit a tree 3 minutes travel time from a major NR maintenance depot and it took 90 minutes for anyone to make it to the train.
I have done the being on a full and standing train with a major problem thing plenty of times and it is very much made easier by there being two members of staff, preferably both knowing what they're doing.