I really don't like driving, steering, and shooting at the same time.
Like the gunner role in the mothership, a second crew member in the srv would potentially have a lot of downtime. No worse than for the mothership though and that made it into the game so it's not a reason for this not being put in.
I have a ship with an SRV bay and an SLF bay. I can multicrew out my slf and if they screw up I (potentially) have plenty of spares. With the SRV I don't, if I lose one it's a Big Deal so to some degree I am happy to restrict that to trusted crew only (ie only me).
What I would really like though, is to be able to have air support via SLF while I'm in the SRV, with the mothership parked (to allow the SRV to return) or hovering nearby (to deploy another SLF if needed).
If the SRV could be resupplied as the SLF can I'd be happy to offer that role to a crew member (or two) while I flew the mothership.