I tend to do this when it comes to core announcements, aka using the username that is the actual name of the forum itself (since these things should be from all staff members and not just shown to be from one cycling through). Otherwise everything else comes from the actual staff member's account, rather than using the generic one to post on the board/handle other matters.
On Crafter Craze I have an account named Admin because I wanted to keep the one I use to post around the site separate. Any official messages or PMs will be from that account only. I'm sure people can figure out it's me but in the future they might not realize it if I can get it to grow bigger. The reason I wanted to do it this way is because on Christianity Haven I have a hard time getting people to see that I've separated my personal views/opinions when giving a Warning. Most of the warnings are for flaming and to me they're straight forward and have nothing to do with my theological position. But there are members who still feel that I'm only getting back at them, which I'm not. You can't call names on the site and that's that. Anyway, I got sick of having to defend my objectiveness and would love to just have an Admin account on there except everyone would really know it's me anyway and not worth it at this point.
It's always so cool when the website is set up right that the very first account name is "Admin" and it's old as flip. I just rename it or use it for other things.
Having a separate account to post is quite nice, it encourages for natural conversations without the idea in people's head that they are conversing with an admin or staff member.
Really depends on the purpose of the forum is, and how official topics, replies and personal messages are done. But I agree that there should be atleast one account made to identify that the moderation action sent, or an official announcement topic that is posted. It helps to make sure that the staff that end up having issues, don't just go and start deleting posts and topics they made that are supposed to help run the place, since the stuff posted isn't tied to their own account.