co-pilot incapacitation: land alone or with a passenger-pilot? Pilots are trained to safely fly the aircraft in case of co-pilot incapacitation:


  "A commercial aircraft can be landed with one pilot at the control and
  Delta pilots are fully trained to do so if the situation were to
  occur." (Delta representative)





  FAA AC 25.1523-1 requires all aircraft to be capable of operation
  by one pilot only from either seat, so just one pilot can safely fly
  the aircraft - albeit under increased workload. (source)


So why would a pilot in such a situation elect to ask over the PA system "are there any non-revenue pilots on board, please ring your call button" and then land the plane together with this unknown person (a military pilot in this case) ?

This happened on United Airlines Flight 1637. Sure, if it turns out right (which it did in this case), it's certainly pleasant to have a helping hand. But there are all sort of things which could go wrong (“At the beginning, I interrupted her flow of operations, but we figured everything out extremely quickly."). Personally, I'd much rather prefer not being interrupted and distracted when I have to concentrate in this critical moment. Why take any risk at all when one pilot should be able to safely land alone?
There are risks either case. There are no hard rules for or against calling a pilot from the cabin for help; it is up to the pilot to evaluate the situation and select the best course of action.

The risk of landing solo is greatly increased workload, which increases the chances of human error. There are quite a few things to attend to when flying an airliner (compared to a small aircraft). It gets worse when you're in busy airspace.

The risk of asking someone to help is the uncertainly of whether this person will do more harm than good. There would be differences in flight experience and operation procedures. However, Crew Resource Management and handling ATC radio applies to pretty much all types of flying.

Historically there had been examples of either choice, and they both turned out fine.