People can be VERY rude during children's programs and sometimes they can be hard to ignore.
Once we had a family ahead of us that recorded their child's portion and then proceeded to play back, with sound, and talk loudly through the next section. When people asked them to be quiet, they got snotty. Another time, in 5th grade, there was a parents' coffee meeting with the staff re: middle school and in a small room, I realized that the guy next to me was having a conversation on his bluetooth instead of listening to the staff. I think it's all a part of general disrespect for others in public. Even when teachers tell people to turn off their ringers, people don't comply. If I have a child at home with the sitter, I leave my phone on vibrate.
My DH is always "on call", too. If he has to take a work call, he quietly slips out and/or notifies the staff that from 10-11AM on Tuesday, he is offline. Every now and then he can "take a meeting" for his kids.
I can see typing a quick note on your smart phone, but there are people who obviously play games (sometimes with sound) and it drives me crazy. Same with the kid who plays DS while other kids are performing. My stepdaughter has reported seeing the lights from phones during a theatre show when the lights are low...and don't get me started on movie theaters.