Re: If it's not luffing, it's helping?
If you're truly "head to wind" and haul the traveller to one side or the other to stop the noise I'd say it's just drag.. if your apparent wind is, say, 20 or 25 degrees off the bow then you're probably getting some drive.
In any event it's WAY better for the sail to stop the luffing, and way easier on the nerves and ears. However if you know the whole trip is upwind and it's not rough, I'd douse the sail. If it's rolly then there's a stabilizing effect that's worthwhile, as long as you're not beating the sail to death.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"