there was a comment made by the op about whether or not there were any documented cases of dive incidents as a direct result of using these handles.
i personally believe the question is not relavent, although i understand why it might be asked.
the real point is that it is clearly an added piece of equipment that is completely unecessary.
not only have tanks been carried using various methods for decades without any issue, but this is an added cost that in not needed. if the handle was easy to attach and remove, maybe some people might find them useful?
perhaps when only diving in open water it may never cause any problem for the diver whatsoever. but it is still an added gadjet that is not required. i could put sleigh bells around my neck and never experience a problem from them being there, but that does not justify having them. even if it is christmas.
it should also go without saying that something like this should never be used if the diver was ever to do any type of overhead diving (wreck, cavern, cave, ice etc). i would be surprised if any shop / instructor would even allow them in the water with it attached in these cases.