Thanks for your thoughts
Zonda, I am not interested in only unusual sightings, I want to make a record of the species in one area regardless of rarity. This means I can gain an idea of the ecological richness of an area, and also learn about the distribution of butterflies local to me, including the most common.
You mention one of our most common (and largest!*) species, the Large White. Studies of migratory butterflies such as this are important in understanding climate change. After all, today's common species are unfortunately likely to be the rare ones in the future - one subspecies of the Large White has already been exterminated by man.
* only 5% less wing area than a Swallowtail
Alexander