It's simply a case of whether you can cut-away the cylinder if entangled/entrapped. Some might say that using a double-ender provides sufficient redundancy of the release to overcome this hazard - but I think if you progress into diving confined spaces you might not agree with that (I don't).
A single bolt-snap can easily jam. If connected metal-to-metal, you can't release the cylinder at all.
Thanks for the answer Andy - logic suggested that this would be the reason, I guess that it should not be such a problem if using standard tank cam bands for the bottom connection because if the bolt snap jammed you could just ping the strap open and slide the tank out or cut the webbing (maybe hard - but possible). Phil