Having a history of 'plodding around the beat' is just part of the plot. It is no more than an indication that the candidate might be suitable for Air Observer. The ability to 'think outside the box, be flexible, have a will-co attitude, have mental dexterity, etc, etc' is all part of a wider testing process that varies a little from unit to unit but ultimately cost each of them a lot of money.
In the end the judgement has [thankfully] to be made by those already on the unit. If ever it was decided that selection was to be made outside the unit [and all things are possible] that would be a disaster even beyond the magnitute of requiring a unit [or a police force] to have a certain 'mix' of bodies regardless of ultimate capability.
ATCO's comment about his take on the average PCSO may well be based on quite a few sightings but it is probably [hopefully!] not too typical. The point though is that they are a money saving measure dreamed up by those above the 'street' ranks to suit the whims of politicians. Getting two for the price of one does not work when they are always seen in groups of two or three and not doing what 'real' coppers do. Look/observe on a 24/7 basis.
Talking to their companion is one thing but looking deeply into their eyes is not quite the same as scanning their surroundings... the rooftops... the passing cars... the ever darting enquiring eyes that always seem to betray coppers in disguise to the criminal classes!
Such behavior is trained in over years and does not simply dissolve with the pension. That is a reason why off-street plodders need two years in before they will even be considered.
Disregarding pre-trained military etc candidates, bringing an ordinary civvie into air observing would necessarily involve additional training or at best a slow aclimatisation into an observer culture.... just the Achilles Heel now being witnessed with the cheap PCSO's on our streets.