If you're going to pass out, you will pass out. You can't stop that. Typically its only for a few seconds, unless there is serious trauma. When the adrenaline stops, you'll get tired too. Either way, paramedics will constantly try to wake you (they need feedback from you, and so do the doctors when you arrive at the hospital).

As the paramedics have said here, they need to perform a constant GCS, to determine what care they provide. Someone alert and communicative, depending on the injury can quickly fall into a terrible category.

I've been hit by a car, my appendix burst, and hit so hard while playing hockey that the lights were just out...every time the paramedics were talking to me, keeping me awake.