I'm downgrading the severity of this... I don't see the security implications. It's an annoyance, yes, but it doesn't compromise the security of the WP install.
There is a plugin that prevents imposters:
http://www.skippy.net/blog/2005/09/08/impostercide/
I don't think this is necessarily something that should be included in core, although I'm open the argument.
I've also altered the summary and description to add the word "masquerading" because the comments are not actually added as the legitimate user... they just appear to be so, to the outside world. The $comment->user_id value will NOT be set.