That still isn't where the vast majority of user time is spent.

If the goal is putting user experience first, and you're concerned about critical bugs, then expend the engineering effort required to reduce the risk of rolling out a bug.

Related aside; Apple can roll out an emergency fix in an hour or two.

I think you're missing the point. How long does it take for that fix to be downloaded? Tons of users will never see it. Tons of users will continue to use the version with the vulnerability.

Here's a good talk on the subject:

http://www.youtube.com/watch?v=MksKaRpWD-o

How is this an argument in favor of an over-all worse user-experience?

If this is a significant concern to you, then prompt or enforce upgrades in your application.

Because it's their app store.

To clarify, I was referring to Apple's policy regarding emergency fixes for 3rd party applications.