New Users - Verify Email?

Why not at least require new users to verify their email before completing the sign up process? It wouldn't eliminate any "real" users, it would only eliminate the greifers who sign up for like 30 accounts in two minutes. It can't be hard to add it, and can't hurt to try, right?

Ah, this was tried a long time ago! (I asked about it too.) So, done and didn't work. Why?

* Lots of confusion thinking the emails were spam, resulting in horrible support load. In other words, way more of a problem than a solution.

* Fake email addresses are so easy to set up and stockpile beforehand; it's not an effective measure against attrition.