You wouldn't leave butyl you are talking about urethane, or what the factory used which was like urethane basically. You cut a layer off the urethane with a razor to create a "fresh" urethane to apply more over before setting the window in. And of course this is when you don't have a reason to remove it all like to properly clean the pinch weld of rust or loose paint or what not. But you would never cut the butyl, that is a whole different animal.
As far as the urethane over butyl, it isn't for cars with air bags, there is no car made that I know of that used butyl on the windshield. It was common and I have done a crap load of them where I used butyl, 60's and 70's cars mostly. But as in any modern unibody the windows are a structural part of the body! So the urethane is bonding that glass to the body to make it a part of it. Butyl tape doesn't even come close to doing anything like that, it is VERY weak.
The only butyl used these days is in bolt in quarter glass and back glass applications in some vans and trucks. What they have is a plastic body around the glass, they call it "incapsulated glass". This plastic piece has studs sticking out of it that stick thru holes in the body at the window mounting area. The window is being held in by those studs and the butyl. Slider rear truck windows are often held in like this.
By the way, not that it means jack but I have an ICAR certification for glass, all it means is at one time I sat thru a class on this so I am repeating what I learned there.