Hmmm, well, personally, I'd say it can't repair bongs, be reliable, adequately seal ducts..i dunno, there's a lot of things it cant do that well. I find metal tape to be much more effective than duct tape and generally use duct tape now only for less critical projects. Every time duct tape has failed me, I have switched to metal tape and been completely blown away by the results, but alas, i digress.
For bong repair you either wanna look for epoxy, or a glass repair glue. Once again, dependent on your personal contacts and the type of break (angle, location, etc.) most skilled blowers should be able to repair it, leaving a warp in the glass, but creating a much more functional and reliable piece. If you can't find a good glue or blower, get a new bong. Seriously, I've seen the kind of damage glass can do to people (especially once they're stoned/blasting off) and it's really not worth the risk, especially given the purpose of your device. It's a real shame to watch someone slice open their hand due to initial stinginess and one tiny moment of distraction whilst using their bong. Part of the beauty of glass smoking art is its impermanence. Accepting breakage as part and parcel of glass smoking apparati is a must for any smoker imo, it's just another one of life's many cycles.