Prodigy
Call me naive, but I think its more likely that Ryanair need to have clear rules that can be applied easily across different handling agents on their network.
To make their business model work, they have to process pax as efficiently as possible - building in the flexibility of a debate over the validity of ID does to achieve this. (Supervisor time costs too.)
Also, when dealing with personnel who are often transient (check in agents), thus requiring ongoing recruitment and induction, these simple guidelines aid learning and retention for people who may lack prior experience.
e.g. I had to politely point out recently, to a new and helpful check in agent handling another carrier, that the cabin crew would not be best pleased if she allocated row 1 to my small kids and I; she had completely forgotten the regulations about exit rows from training, which I guess is understandable if you are taking in a lot of new information and struggling to get up to speed in a fast moving environment.
Of course, if the scenario you describe occurs, Ryanair will benefit, but my guess is that keeping it simple across the whole network is the primary driver for the ID rules.
BTW, I have consulted on airline ground ops, so my speculation is based on some foundation.
Whatever the reason, the outcome is clear with low cost carriers - transgress at your own peril - but in fairness they state this clearly in the booking info and if people do not read it or choose to ignore it, then I fear the subsequent argument is rather one sided.
At the end of the day, quality of customer care is a given for any business model, but it will vary from one to the next.
FR compare very well to Wagn railway (where even 1st class ticket holders are not guaranteed a seat on the London commuter routes, but not so well to BA. Lufthansa etc!