Muggles' Guide to Harry Potter/Magic/Alohomora

Overview
Alohomora is a spell that is used to opens locks on doors and windows.

Extended description
We are first introduced to this spell two weeks into Harry's first school year, when Hermione uses it to open a door, so that they can escape from Filch. Throughout the books, when a locked door is found, Hermione, if she is available, is called upon first to attempt to open it; it seems that she is the most practiced at this particular spell.

Not all doors will respond to this charm, though; for instance, in Harry Potter and the Order of the Phoenix, Professor Umbridge has counter-charmed her door so that Alohomora will not work, although the knife Sirius gave Harry will.

Analysis
One has to wonder about the usefulness of having locks at all in the Wizarding world. The unlocking spell is so simple that a 12-year-old girl can work it, and it is apparently taught as part of the first year curriculum at Hogwarts; and artifacts (such as Sirius' knife) exist that will unlock even those locks that are charmed against the unlocking spell. From a Muggle point of view, we must assume that an ordinary lock, in the Wizarding world, is similar to a Muggle Yale lock that can be slipped with a credit card; basic security and not much more. If the Alohomora countercharm is used, the security is more that of a bolt lock, with Sirius' knife being akin to more specialized lock-picking gear; presumably, that knife would be controlled by Ministry law, or purchased outside of the legal system. And finally there are high-security locks, such as those at the Ministry (one of which actually melts Sirius' knife) and the vaults at Gringotts.

Questions

 * 1) Hermione finds this spell in The Standard Book of Spells, Grade 1, so why do wizards bother to lock doors in the first place if such a simple spell can open them?