IMO, it would be unspeakably rude to essentially tell guests "we don't trust you to behave like adults so we're going to ban your phone usage".
I can understand the irritation with phones, but, really, requests can be politely made, orders cannot be politely given.
(and there have been times in the last few years where a contact number wouldn't have cut it--I was the contact person for 2 relatives in hospice care, i was the 'emergency contact' for family that was travelling in a relatively risky overseas location, etc--and the important people knew I was reachable 24-7 for these purposes. Contact everyone and saying "oh, everyone please know that next Saturday between 8 am and 2 pm, you should reach me at this number instead" would have been an hassle of epic proportions.
People handled these things before cell phones but, honestly, I've currently planned my life around having the phone on and available--I'd have done my planning differently if the phone wasn't available.)
I don't see why it would be a hassle of epic proportion; all you'd need to do is put an outgoing message saying "Dawbs is unavailable until 2 pm. If this is an emergency and you need immediate attention, please call {number of venue}."
*snip*
Because it's not always that simple.
The person calling from overseas, if it was an emergency, would probably be the individual--for whom English is a second language (we communicate OK, but not through messages)--and figuring out the hassle of international calling from a developing country where one doesn't know the language is going to be nigh impossible when we're working with translating numbers.
And part 2 is that I'm not going to count on a 3rd party individual to be able to tell me whether my grandfather is 'kinda' doing poorly or 'very much' doing poorly. I'm not going to risk missing a call from my grandmother who found phones difficult on a good day and who had difficulty hearing messages. I'm not going to risk there being a delay in messages being given to me, a delay in me being able to make a call back or there being a miscommunication within the message.
Like I said, if I lived in a world without cell phones, at the time when things were that hectic, I would have made alternate plans--and I would have (and, as a matter of fact, even with cell phones, did) turn down a lot of invitations. But the plans have been made assuming cell phone availability; I'm not going to risk not being available, given the circumstances.
And the implication that all cell phones have to be banned because some people will play angrybirds during the ceremony is rather like saying no alcohol should be served at weddings because Uncle Phil has a tendency to get drunk.