In particular, I'm interested to know what proportion of the year a FR 737 spends on the ground when it's neither being used for flying passengers for profit nor undergoing any kind of inspection / maintenance or other commercially useful duties. This number is probably rather more important when judging FR's capacity to make optimal use of its fleet rather than just time in the air. Presumably it's better to have an aircraft doing naff all in January, than grounded for maintenance in July...
The rotation of aircraft means that you don't just park up 100 and wait until spring time but as you said you carry out any maintenance and constantly rotate in and out so that every one of them gets used.
Given the recession within EU at the moment it makes sense to take capacity out of the system rather than aiming for fleet maximisation.