The even more obvious reason is that users who are logging in with multiple accounts will end up voting multiple times on the same comment. This is obviously not good.
I don't do that, but obviously you only have my word for that. Thank you for elaborating.
I've just launched WorldviewNaturalism.com, which is intended as a simple "landing page" to be used for introducing your friends to scientific naturalism. Many of the recommended readings linked there are written by LWers. Enjoy.
(This is a very old personal project on which I've spent a few hours per month, and it is not at all associated with the Singularity Institute or the Center for Applied Rationality.)