Theft deterrent? Who would want to steal a bike that looks like it's held together by duct tape?
Probably just some idiot who had a couple rolls of duct tape laying around with nothing better to do.
That said I get the same impression when I see cars with cracked fenders / bumper covers or rust holes taped up with duct tape. Not sure if the owner is too cheap to fix the problem right or truly believes tape will make the problem go away.