Sorry Mix
"Sure, I wasn't saying there weren't other costs, but the club commited to these costs on an aircraft basis ... whether or not Student X decides to have a boozy lunch and not turn up will have no effect on the cost of insurance, hangerage etc, as the club will be paying that irrespective for as long as they are the owners of the aircraft."
Okay, I'll book all your aircraft then naff off for a boozy lunch somewhere.....meanwhile, there are many other customers who would have flown the aeroplane had you not said 'I'll book that'. The more hours the aeroplane works, the less per hour the insurance/hangarage/parking etc costs (simple economics)...if it's on the ground when it could have been flying then it's losing money. Simple as.
Especially at weekends, you can find it difficult to get a slot/instructor/airplane so why should others suffer, just cos you've broken your promise to use the aeroplane.
Meanwhile, back in the real world.....usually we only enforce this rule for persistant offenders!