"To me it makes it hard to use a motorcycle as a source of transportation if I have to wear ATGATT. Let's say I want to go to a movie. What am I supposed to do with ATG once I get there? Take up 3 seats with it?"
You could always get a helmet lock (there are several kinds) for your bike. There are even cable locks that let you attach your jacket to the bike, though the jacket shouldn't be too hard to either wear in the theatre or hold on your lap. (Same applies to your riding pants/boots. You might look a bit different, but cool nonetheless.
As far as ATGATT downsides, the main issue AFAIC is the inconvenience of "suiting up" to ride as opposed to just jumping in the car with no special preparation, and then the need to "de-suit" when you get to your destination if staying in your gear isn't an option (eg, riding to work).
Secondarily, the gear gets warm when you're not moving, of course.
Personally, I'm uncomfortable not
wearing ATGATT nowadays, though I used to wear NOTGATT (none of the gear, all the time) in the '70s, not even a helmet, when my motorcycle was my only transportation.
Older and wiser now, and ride just for fun.