Even sillier question - why use a biowheel at all?
Because it makes some peole feel like they are improving the health of their tank putting gadgets on it that don't really accomplish much. Walk into any high end stereo store on a saturday and note all the males customers buying high end cables and other gadgets that don't do anything either. Same part of the brain in play
If you noticed, when you pull the biowheel off your tank nothing much happened. The reason is that gravel and other media inside your tank will out-compete the bio-wheel as bacteria hosts. So, this falls into my category as gimmicks that don't do much, but don't harm either. The good thing is that HOB style filters introduce top/down current flow into a tank, and this is WAY better than bubblers or most cannisters. Note that if you are fussy about C02 levels in a planted tank than HOB water flow might be counter productive for this reason. For breaking up water strata though in a bigger tank, HOB filters, even empty, are terrific water movers and preferable to power heads.
The one thing that does bother me is trying to get somebody to rationalize with me the justification for even wanting to move the critical biological filter from inside the tank (where it is most stable) to to a plastic appliance hanging on the tank that requires AC power to operate. This makes no sense. It's like, a healthy person saying they actually want to have a colostomy bag.
FYI - those of us who are experienced on the marine side don't promote bio-wheels either. The difference is there is still an entrenched marine community that uses wet/drys that utilize the same principle as a bio-wheel. These are ironically the same people that seem to whine the most about tank crashes and high nitrates anyways.