Muggles' Guide to Harry Potter/Magic/Impervius

Overview
Impervius (spelled that way in the books) is a spell that is used to waterproof items. It seems to work by actually repelling water, rather than preventing penetration.

Analysis
This spell is first used by Hermione to waterproof Harry's glasses in the Quidditch match against Hufflepuff in Harry Potter and the Prisoner of Azkaban; this match is played in a pounding downpour, with lightning and gale-force winds. Harry is unable to see the Snitch because the rain is streaking his glasses and distorting his vision, and this spell clears his glasses so he can see.

A relatively minor spell, it is used in several other Quidditch matches, but is not used elsewhere.

Questions

 * 1) Being waterproof seems such a useful thing; why would this spell not be in much more general use?