Why is it useless? Well, if you submit your email address, it will either be displayed on the web or it won't. If it is, then, sooner or later, a spam spider will come along and harvest it and the address will be inundated with advertising for dodgy debt-elimination/penis-enlargement schemes until the end of time. If it isn't, then why bother collecting the email address? It's not actually used to send a password to the user or anything like that. It's like the "Anonymous login OK, send email address as password" thing FTP servers send, only even less sensical, as back when FTP came about, some human may have conceivably looked through the email addresses thus entered and gotten information out of them. (And that's not counting the sites which don't show addresses in their HTML but leak them in their RSS feeds, but I digress.)
So you decide to write 'none' or something. No dice; Movable Type has ways of making you submit something that looks like an email address (or, to be precise, that is within a Perl regexp's distance of one). Not that it does much to defend the MT email address collection system's integrity against spam-wary users; something like 'a@b.cd' fools it. In other words, the enforcement mechanism is strong enough to be annoying, but utterly useless against someone determined not to comply.
It's not a huge effort to remember to type dont@spam.me or something in every time you post a comment to a blog, but that's not the point. The point is that there is no logical reason to enforce this requirement, and a very good reason for not entering one's real email address on any such form. It is also impossible to verify whether the address coerced out of the users is valid or just looks like it might be. As such, the decision to require email addresses in comment posting forms is bad design, and does nothing other than annoying users and filling databases with garbage.
</RANT>
I wish it didn't collect emails too - a couple of times I've clicked the "Send Notification" button instead of the "Save" button accidentally - I've stopped it just in time, but it almost sent out dozens of blank emails to everyone who's ever left a comment.
thanks for sparing me from that!
Ah, in theory the 'blog author may be interested in replying to your comment in private email, or contacting you for XYZ reason, or...
Which explains why they might leave "Your email:" up even though they won't be publishing the address. 'sn't explain the requirement for it, though.