I'm a 911 police/fire/EMS dispatcher. Just some thoughts on this in response to Marty's comments...
(not a personal attack by any stretch - I'm just offering some insight from my side)
In our department, we have 8 wireless-911 (W-911) trunks and 8 landline-911 (LL-911) trunks. We field every W-911 call in the county. If all the W-911 trunks fill up, they roll over to LL-911 trunks, and when those fill up, it rolls over to PBX lines. In other words, it is impossible to get a busy signal at our center. There is always a backup plan and always someone available and trained to handle the situation. We have dealt with major train derailments, 100-year floods where hundreds of people required evacuation, marsh fires, etc. Never had an issue.
I'm not supposing that all 911 centers are the same as mine, just saying that we have a failsafe plan in place for that.
Anyway, maybe you're thinking "sure, the phone lines roll over, but what if the dispatcher is taking a non-emergency call on 911, while one of the rollover lines is ringing?" Marty, this hits your "what if your family missed out in a real emergency" question.
In our center (and I assume most of them), this guy would get put on hold if there was another 911 call coming in because we understand the need to prioritize calls. So when someone dials 911 for keys locked in a car, barking dogs, parking violations, or other non-emergency things, we handle the call just as if they would be calling on a 7-digit non-ER line. But in the back of our minds, because we know our current caller is not in danger, we know they get put on the hold if it requires it.
In an ideal world, yes - 911 would be for emergencies only. But if you heard the dumbass calls that I sometimes get on 911 (asking when trick-or-treating/July 4th fireworks/parade are, asking the police to give you a ride to work, the number for the pizza place, etc), you'd probably look at this kid and go "No big deal."
Hell, the two worst calls I ever took were on "non-emergency" lines. One was a gun fight in one of the Metro Housing projects. The other was for a lady who woke up to find her 15 year-old son overdosed on inhalants, and insisted that I help her with CPR instructions even though it was painfully obvious he had been dead for at least 8 hours.