I'd ignore that: Unless you were doing it every single day, then it would be classed as an unfair contract term and dismissed in court. As you say, there's plenty of reasons you might not always have the car in the garage overnight and as long as it was the exception rather than the rule, it would not be reasonable for the insurer to reject a claim.
Not sure about this, it is worth checking with your own insurer. My next door neighbour had his car parked half on the road and half across his drive (because he'd had visitors parked on his drive) and left it in this position overnight (he insured the car as being on the drive overnight). Unfortunately the car was burnt out overnight, and the insurer (not sure which company) refused to pay out!