Fold a closed zipper over and stomp on the crease, and you can probably force the teeth apart, i.e. create an opening away from the slider, which can be a royal pain to restore to normal working configuration. Or the teeth might break. Usually not an issue with tents, since you're rolling the fabric or doing gentle folds, not sharp creases.
It's also possible that design of the base of the teeth would make a difference in how sharply you can fold the fabric. Newer zippers might be more fold-resistant.
having it zipped before packing means one less thing to do if, say, I have to set up a tent quickly in a drenching rain.
Ditto that. I keep the fly attached with the door closed, so the inside stays dry while I lay out and stake the tent. I can even raise the tent without removing the fly, though it's not easy.