All frame spacing does is allow a hub of a certain width to be easily inserted. If you have a rear dropout spacing of say 126mm and you stick a 135mm mountain bike wheel in there by pulling the stays apart to allow the wheel to fit in there, it will work just fine with no problems other than your fingers getting sore from holding the stays apart. Spacing won't affect the ability of a wheel to pop out or not no matter if you have horizontal or vertical dropouts. The pressure from the skewer properly locked in place is what holds the wheel in place. An improperly locked skewer is what allows a wheel to pop out. Doesn't matte if its a vertical or horizontal dropout.

In a nutshell, frame spacing basically just makes it easy to insert or remove a wheel, it really doesn't do much more than that. A convenience is what it allows.