Aside from:
Seems like a sweet tool for spies in your Guild.
and
It's none of your business when I am on. If you want or need my help with something I expect you to extend me the courtesy of asking me in advance.
I can understand the privacy issues, but for some parts of the game (GBG, GVG? and GB contributions, especially in sweet spots) I can see where it would be a big help to see who in the guild is online to coordinate efforts.
You see a bunch of Guildies are OL and what?
You still have to message them to ask them to do whatever it is you want them to do.
All knowing they were online will do is frustrate you and prolly generate a lot of 'You were on, why didn't you respond / help?' messages.
While you may or may not send that message, you're sure to think the question.
Nothing good will come of that because even if they answered (would you? The 10th time you were asked?) you'll never really know if they were purposefully ignoring you or there was a legitimate reason to not help you.
Save yourself and others the angst. Stick with coordinating the way you do now, sending out a message asking who's online and can help.