I have never tried on a dress which did not have a crotch snap. I would guess having them is more common than not having them. All my dresses except one have had it.
The one dress that doesn't is very, very easy to get in and out of - it has a low-cut back and no zippers, just a nude strap across the back. The company which made it for me, didn't put it in because the skirt has a slit and they thought a snap could be visible. I prefer that particular dress that way, and while I was miffed at first when I received it, I quickly realized how easy it is to get in and out of, so not having a snap doesn't complicate bathroom visits, and it feels much nicer not having it.
Now, if any of my other dresses did not have a snap, using bathroom would be a major undertaking, because they're a lot harder to get in and out of. In particular, I had one, which I couldn't close on my own, I always had to find someone to help. Also, opening and closing zippers has its own hazards. I had the dubious pleasure of having a zipper break as I was putting on the dress 1 hour before I was supposed to dance, and I had no choice but to dance with a bunch of safety pins on my side. If I have to pick my poison, I'd rather risk a snap coming undone while I dance than having a zipper break during a bathroom visit.
Also, FWIW, I only had one crotch snap mishap with high-end dresses I own(ed). I had multiple issues with the low-end ones, they just did not use good and sturdy hardware in that area.