My county protocols dictate that I, as a basic, need to take it immediately upon suspiscion of head or neck injury, and that I don't get to release it until a patient is backboarded, not just collared. Simply telling them not to move or putting on a collar isn't enough. C-spine can be cleared by a medic, but they're not going to clear it if there's a reason to suspect head or neck injury.
As for why it might take awhile to get a backboard, it depends. Maybe we were dispatched to a call that had nothing to do with trauma with a patient that's well away from the nearest road, and we need to send someone back to the road to get the backboard. Maybe we're waiting on lift assist. Maybe extrication is taking forever.
As for why me, in particular... I'm often the most junior member of a crew, so tasks like endless c-spine, holding the vomit bucket, and the like typically fall to me. As such, I'm not in a position to dictate the best use of resources, or dispute our protocols. If we have enough people on scene, I can trade off once I get tired. But that isn't always the case.
As for position, it depends on the situation and patient. But it's most often sitting in the car seat behind the patient and reaching over the back of their seat, kneeling, or sitting cross-legged. I'm not having any trouble with staying in the positions I assume. It's my forearms and hands that are getting tired.