Air forks have two kinds of service - a lower leg service and full service. For the purposes of this discussion, consider a full service is something done to fix problems like the fork not holding air, damper not performing or repairing a breakage. A regular full service is common for bikes used for serious mountain biking, often done by sending the fork to a specialist.
A lower leg service is the 50 hour one and often done by the rider. For a serious mountain biker it's just another regular maintenance task, either DIY to taken to the bike shop. The reason to do this service is preventative as well as keeping the shock working properly. Not doing lower leg services at all can lead to expensive damage that effectively writes the fork off. Doing them when specified means the fork will perform well for decades. Spring forks also need a lower leg service for the same reason.
If you look at a fork construction, the stanchions slide into the lower legs. There is a seal with a lubricating ring that keeps the dirt out of the fork and keeps the fork lubricated, reducing stiction (the force needed to get the fork moving) - ideally forks have no stiction. There is usually a bath of a small amount of oil to keep things lubricated in each leg.
If you do not service a fork, two things happen - the lubrication does not get to the ring, and it dries out. This is bad because the fork's performance is severely compromised, and it wears out the surface coating of the stanchion. While very hard, this coating is also very thin. Once worn out, the stanchion quickly wears down and destroys the the seal. The second thing that happens is dirt gets past the seal and mixes with the oil creating a grind paste, wearing the stanchion, destroying the seal.
The service involves removing the lower leg, cleaning everything, replacing the seals if needed, reassembling and adding a small amount (typically 5-50ml) of clean oil with a dab of your favorite fork butter for good measure. Its a 15 minute job for someone who's done it too many times before and has all the right oils and lubes in a well organized workshop.
The question that needs considering is what interval -50 hours, maybe push it out to 100, or 500, what about 1000. I tend to do it when the fork feels it needs it - probably closer to 100 hours. The type of riding makes a difference - sealed roads in dry weather without too much dust you could push it out. Wet or very dry, dusty weather I would not push it too far.
If you lock forks regularly, you really need to keep the servicing going. Few MTB forks true lock with no movement. The small amount a locked fork has concentrates the wear at the mid point of the fork stroke.
@Andy covers the idea that a suspension fork is not the best solution to the problem of cobblestones and pot holes. They are heavy, expensive and require significant maintenance. Talking specifically to Air fork - I suggest they are less suited to commuting than a coil fork, but not because of servicing requirements (which coil fork have the same requirements to keep them performing well), but because they have more things to go wrong. You need to adjust them correctly to get best performance (air pressure, damper settings) or you wasted a whole heap of cash, and they need regular air pressure checks. If the seals go, they need to be stripped and serviced (trust me, you will very quickly get tired of pumping the shock up before every ride).
I might consider one for a commuter - I already do my own fork servicing, have a fork pump and love to play with pressures to see if I can get it better. If I wanted a bike I could ride, doing nothing more than banging some air in the tires and oil on the chain once a month, I would not get one with an air fork.
