This first of all this should be an option for admins to decide what is best for their site. Regrettably, its all to easy for any community to fall into the mob mentality of "who's first".
X user signs in, sees no one online, signs out without even reading... Except unknowingly to X user there is 10 invisible people, who also cannot see each other and so hover to see who comes in, notices no one, and so signs out.
Now obviously for some communities privacy is a top issue, so this should only be an option, and by default (stock install) members should be able to freely make themselves invisible if they choose.