It ain't all that great for rehabing an injury either.
Ok, for individuals for whom normal weight bearing cardio isn't an option. Honestly I usually see PT's use things like this for elderly patients that are in wheelchairs and are not coming out of them.
Maybe if you were trying to work around a leg/ankle injury? Might have done me some good one of the times my ankles got done in by a toe hold or heel hook, but I am thinking punching and pummeling drills are going to give a better carry over.
Honestly, benefits of using a hand bike vs an exercise bike vs some other form of slow steady cardio has to be pretty low on the priority list. I doubt I could ever get dialed in enough to even be able to swear if it made a real difference from one to another vs just the whole more cardio is better thing.