Stealth Settings in yahoo! - A Big Problem

Just a recommendation from me when you are chatting over yahoo!
Never tell that guy/gal the names of other ppl u r chatting with.

The incident that happened to me:
I was chatting with 'A' and 'B' simultaneously.

'A' : So, what else.
ME: Nothing much. Hey, i am chatting with 'B' as well.
'A' : Strange. 'B' is in my list. But, i dont see her online.
ME: oh oh!!!

ME(to B): Hey, i am chatting with 'A'.
'B' : Oh is it! Lemme ping her.
'B' : Hey, 'A' isnt repsonding. I wonder why.

(Dumbo, its because, i told 'A' that you were online. But, YOU, were invisible to 'A'. That spoilt it all)

Damn, this yahoo! msgr. Why did they have to provide this stealth settings.