general body work question

I sometimes get farmed out from the shop where I work -I go down the street to a body shop and notice the guy tearing off accident damaged sheet metal with no welds and ask him about it and he says the factory "glues" a lot of panels now and it"s as strong as welding. That gets me thinking ( OH OH !) : why can't you flange a seam and glue quarter panels on? or for that matter , glue console brackets ,etc ? I was briefly talking to another guy there about removing the wheel chair brackets off the roof of my car and he said never weld up the holes-just drop the headliner , clean the area down to bare metal and use "All Metal " to fill the holes- does that sound right ???? I'm a brake/ front end/ mechanical dude so any advise is really helpful !!!