Here's a reason I like to be set on lower powers in very thick stuff. Sometimes I spot a possible piece of a deer through foliage and then, when looking through my 8x binocs to sort out what I'm seeing, it can be hard to find the same stuff, the same opening or whatever, because things look so different at 8x than at 1X (IE, naked eyeball). And when putting a scope on an already-ID'd deer in thick stuff I've had similar difficulties set at higher powers.
Obviously, part of what I'm describing is simply my own limitations and I try hard not to assume that others labor under my limitations; they may not. But speaking strictly for myself, the transition from naked eye to magnified vision can be jarring and I have a much easier time at low powers, when the chit is thick and you can only see parts of the deer etc.
As to Bob's comment on swinging, hmm... that's interesting. Thinking how I've done things I do believe I've swung on moving deer but on elk, I've done as he said and ID'd in one opening, then prepared to shoot in the next opening. Never had a shooter in that circumstance though I've watched many elk that I didn't have a tag for drift through openings...
