Not sure...
Not sure of the likelihood that it will “snap your neck”, but I believe that a closer fitting helmet is safer. On the other hand, this woman came off at 120MPH in a helmet she describes as two sizes too big and it saved her life:
http://rockthegear.wordpress.com/brittanys-story/. I would count that more as a fluke though, like people who crash with no helmet and walk away—doesn't mean I would ever ride without one.
Anyway, more to your point, tolerance of a tight-fitting helmet is probably 100% unnatural for most people, but one may be be able to develop it. When I bought my first full-face helmet I thought it was snug, but I couldn't imagine anything tighter. Break-in obviously loosened it up some. A year later, I went down two sizes and I'm loving my smaller helmet for the more agile feeling and reduced wind drag. Is it tight? Yes, but it doesn't give me a headache. After about an hour my ears feel a little squashed, but I reckon on longer rides I would take breaks as needed to relieve issues like that. Glasses are also a bit of a hassle, but doable.
Another huge issue is head-shape. If you're trying on helmets with the wrong shape, they will never be right. I could actually do with a slightly longer shape—tried some on and they felt amazing—but my current helmet won out on price for the features I wanted. It's within the acceptable range; anything rounder would be too round.