I've just implemented it and pushed it to the repository since people have requested this. I guess if your site doesn't get much traffic you are worried that admin traffic is inflating the numbers but once you get a lot of traffic it doesn't really matter.
Myself I would not use that. It seems arbitrary to not track that data since visits by the same users who are not logged in but happen to be in those roles (if they were logged in) will still be tracked, so you are only not tracking them when they sign in.
Google Analytics provides lots of features such as advanced segments that would allow you to filter out things based on the path of the page the ip address and custom variables, we already pass a custom variable for admin users and others have used this as mentioned on this thread.
My feeling about it is its best to track everything, you can filter out things you track but if you don't track it you can never get that data. What if you want to make custom reports specifically about how your editors are using the site? You won't be able to since you won't have the data.