I dont know about this, I have read several accounts of miners that have been trapped and suffocated and the lone surviors always say they others just went to sleep and died peacefully with no complaints.
As far as wacking them not everyone is comfortable with that or good at it. I have seen an assumed dead rodent wake up after a wacking and go psycho, not good for a snake in harms way. Separating the spinal column is more effective but again not everyone is comfortable with that as well.
The nice thing with CO2 you can kill many rodents at one time while you can only wack one rodent at a time. I have CO2ed many a rodent and they go quiet quietly, peacefully, no mess, no fuss. Its alot easier when you have large collections to feed.