It's a definite balancing act. Send the spare aircraft to recover one set of pax, or use the spare to recover the aog's next rotation I.e save 2 sets of eu261 payments.
Jet2 are far better at most at recovery there's no doubt but yes its primarily for cost saving but with the secondary effect of keeping the punters happy. They also have a large number of older fully owned aircraft that they can afford to have sat around on standby. It will be interesting to see if they have the same approach when they have a larger percentage of newer more expensive aircraft.