I don't understand.
You're signed in as online - someone sends you a message.
...
Then what happens? How does this impede video conferencing? You're sorta skipping some middle element here I feel.
Depending who that someone is you might have to interrupt what you're doing and respond to them. It also becomes an annoying notification center item.
Not the same as a phone call since people intuitively understand you might not pick up. Not the same as a text message as people intuitively understand they might not get a response right away. For many people, especially older, being online on Gtalk = being in front of your computer and available to chat.