If the stim pistol were permitted to self-stim, then I think that they'd need to take a page out of Killing Floor's handbook. In that game you could heal yourself with your stim tool, but it was a weak heal. The heals were far more potent when used on teammates.

So translated to helldivers, self-stimming with the stim pistol wouldn't be a full strength heal. Instead it might only be 10-20% of a normal stim.