I once had the screw holding one of the arms fall out during a flight so yes, a spare pair is necessary. CASA probably got wise to people just buying a $9.99 pair of reading glasses to show as a back up and decided to check that they were the correct prescription.
Back in the 1990s the opthalmologist doing my initial eye exam told me that photocromatic lenses weren't allowed because a DC9 Captain had a heavy landing after flying on top in bright sunshine and then descending through low cloud which didn't allow the lenses time to adjust before arrival