> I never understood publicly defacing a site like this, if you're technical enough to access a domain with a fair amount of traffic, why not inject some script that'll give the visitor a drive-by download?

Because the point isn't to cause damage to the user in any way (usually), but rather to (some combination of) 1) show off your skills, 2) embarrass the site owners, 3) get a message out, 4) have bragging rights. It's generally a fairly non-destructive practice, just silly.