No one questioned the babysitter's competence. They're your children -- you're going to want to call and check to see how things are going.
Yes. If you suffer from high levels of anxiety and don't trust your own decisions about who to trust, this is true. Many people, particularly Americans, are this way. It is not true for every parent, however.
The vast majority of the time consumed in that process is simply getting out from where you sit/stand and back, which is the same in the "unjailed" phone scenario.
Unless, of course, you were just going to go ahead and conduct that call right there in the venue anyway.
It all reminds me of DRM causing problems for legitimate, paying customers when the DRM should only be targeting those who go outside of the rules. That's where someone else's comment about treating everyone like adults and kicking people out who break the rules seems like the best bet, although they also admitted (and I agree) that this system just doesn't work as needed, either.
A lot of the time your mates aren't looking at the phone at the right time, or there are so many people that you don't get good reception. Looking up and just finding people works pretty well.
Surely not every time you go to the loo, if it's just the once it's hardly much inconvenience.