This started going inside Google quite a while back. My feeling then is about the same as now: feel free to say "hello" to me if we happen to be on the same IM system at the same time and you are "opening hailing frequencies", so to speak. I might not be there. I might not have a chance to chat right then. You can find out both without a serious investment in time pre-typing some huge thing which might be a total waste anyway.
It's a good opportunity to get some idea of how quickly things will move, and whether the other end is willing and able to have a solid chat at that time. This applies to being either party in the chat: the asker or the asked!
What's far more annoying is when the [typing] indicator goes on and stays on for a couple of minutes, at which point you are "rewarded" with a wall of text. By then, they have filibustered the chat, and are probably three steps down the wrong road if they got off on the wrong foot, and they need to be backed up and pointed in another direction.
If they start small and evolve it as a natural conversation with back and forths, then a whole lot of needless typing can be avoided.