Helmut Question

Please don't turn this into a thread of the pros and cons of helmuts....

First of all, I never wear them. But last night was only my second time out this season, and after taking a really nasty fall on my first run, I decided to try out my buddy's ProTec for my second run. The size was perfect, and I adjusted the chin strap to fit me, but after a couple jumps I landed funny off my toeside, and when my head hit the water, the helmut got 'pulled' and the chin strap ended up sliding up on my adam's apple, which gave me an abrupt choking sensation after the fall. It knocked the wind out of me for a little while, and immediately made me think of all those helmut posts I've read on WW!

Anyways, my question is this: if a helmut fits snugly on the top of your head, do you lose any benefit of keeping the strap unbuttoned? I was thinking that it will stay on while riding, and help in a fall, but this way I wouldn't experience the pulling sensation from the strap in the case of certain types of falls. Anyone think this would be a good way to go, or would it ruin the purpose of wearing the helmut?