Mostly that, any dent big enough to compromise the structure will only be worse after it's been flexed back into the original shape. Also glue isn't strong enough to pull dents out of small diameter tubes, so it wouldn't work anyway. You could pull out a dent by brazing a rod to the middle of the dent and pulling that, but you wouldn't want to. If it's too big to fill, you want either a new tube or new frame.
Mostly that, any dent big enough to compromise the structure will only be worse after it's been flexed back into the original shape. Also glue isn't strong enough to pull dents out of small diameter tubes, so it wouldn't work anyway. You could pull out a dent by brazing a rod to the middle of the dent and pulling that, but you wouldn't want to. If it's too big to fill, you want either a new tube or new frame.