Hmmm, I've left it in on several of my answers, because I think it actually looks nicer with it in. And sometimes it is important (for example, when multiple machines are involved, or multiple users).
anthony@Zia:~$ ssh haruhi
anthony@Haruhi:~$ sudo -i
[sudo] password for anthony:
Stripping out the
user@host in that would make it much less clear.
And, really, I dislike editing the output in examples when not needed. I feel they should, as much as possible, reflect the output the person relying on the answer should actually expect to see. I agree some editing is needed (e.g., in my example above, I cut out the login message, and indicated it with ⋮, because its fairly long). But the user@host part of the prompt isn't that long, is easy to skip over, and doesn't make the post taller.
I'd also discourage people asking questions from removing output, except in the case of privacy, because the OP may not realize the line he/she is striking is actually important.