Google offers a portal to thousands of webpages to plagarise from. Hell, Google indexes some universities' lecture notes directly. Though it's possible to just use Google to find notes, I don't see any difference between plagarising from an online journal and plagarising from this website. As you said, lecturers know how to use the Internet, one way or another, the student caught plagarising will be caught and have what's coming to him/her.
This website isn't a special exception to the billions of others out on the Internet. In the case of academic research (which is what I assume a worry to most of you), one would have to be pretty daft to use an online student community as a reference for academic writing. Online journals are there for that. But that being said, I don't see anything wrong with summarised dot points explaining some concept that someone else might have trouble with. A centralised repository that holds information on this topic, which would be far less dubious than some results Google would pick up (I'm sure there will be a few dubious sources there, whether maliciously or accidently, nothing is perfect), is an ideal idea.
That being said, I do understand the cost/effort involved might weigh out the feasability to implement such a repository. However, dismissing the idea, just because it is not feasable at the time, is not always a good idea. Many modern inventions/innovations would not be in existance today if ideas were just dismissed and not dwelled upon.