Most sites don't do this because, in theory, a Man-in-the-Middle attack could be used to spoof the non-HTTPS login page so that the login information can be intercepted.
Though, in practice a MitM attack is a very rare and sophisticated attack. Unless you're on an unsecured network, there's almost no chance of it happening. And if you were to be victim to a MitM attack, I'm not even sure using an HTTPS login page would help. (There have been reports of rogue CA certificates being created by exploiting the use of the obsolete MD5 algorithm by root CAs; as well as stories of wildcard certificates being generated that can be used on any FQDN.)
That said, if SSL is secure (hopefully those exploits have been fixed), then the customer should be safe from MITM attacks if they're using an up-to-date browser and pay attention to browser warnings (sort of a big "if" considering many big sites don't even bother to use proper SSL certificates). And users do feel safer when they see the little lock icon on their browser, so that may be a good enough reason to use it, as there are few downsides to using HTTPS on the login page. The slowdown should be negligible considering the overhead of SSL encryption is minuscule compared to database or scripting overhead on most applications.
