Earlier quoted context omitted.
But they would probably see abc.foo.com as different from xyz.foo.com
The www. prefix is a special case for people; it literally means "its the w orld w ide w eb" or "its the internet" to them. Technically, the subdomain is irrelevant. But, most people don't know this. They don't know how the internet works and (I'd suspect that this is true in more cases than you might think), think that www. is needed to get to whatever site they want to visit. Of course, this is all assuming that th…
Try this link: http://www.news.ycombinator.com for example - this is what happens to most addresses not prefixed with www. This means that hiding the www. would mean waiting to work out that the site doesn't exist (or the connection is down) before trying the non www. version, and worse that you would get the wrong site if www.foo.com and foo.com were distinct. Hiding the http:// doesn't have such massive implementation issues.