I think the idea was that the message should be composed as one submission rather than a series of short comments strung together.

Alternatively, if you're in one of those not-available states, most IM clients have some kind of facility to publish that availability...

...and often people still send me the "Are U busy?" message despite my status being busy.

I've started using the "Do not disturb" option in my Office Communicator when am in important meetings. Using that option on the communicator makes it impossible for someone to send me a message. Any attempts explicitly fail on the sender's end with an error stating the obvious - The receiver has set "Do not Disturb"

Why? Does your chat client beep differently for longer messages? As long as it all fits in a single message, what's the difference?

i think the point was to avoid incessant beeping due to multiple short sentences.

I have seen the embarrassment on people's faces when in a meeting and their IM app starts beeping like a time-bomb after someone has cut the wrong-wire.

The sender sending a short 1 liner and waiting for acknowledgement before proceeding would prevent this. (results in a single beep if the receiver is busy)

Its also avoided if the sender simply sends their entire query as a single message. (again results in a single beep if the receiver is busy and doesn't reply)

In my opinion, that's a bug in most chat apps. Multiple messages in succession from the same sender should only beep once. But I realize most people use restrictive apps, so they have to adapt to them rather than the other way around.

if you're in a meeting/trying to concentrate/etc, then why is your chat status still set to available?

Because I am available for certain questions. I am available for "quick" questions which only I would know the answer to, and for critical questions. I want to defer "easy" questions which the questioner can easily find the answer to, but is lazy or incompetent. In my role, I will get questions from downright simple "how do I reset my password", or was a certain move-to-production done; to some really critical ones, or, where others move on my go-ahead. The move-to-production query means different things based on who is asking. One person asks only because a end-user or their manager wants to know the status. Another person is asking that as a start of "we are seeing something strange in production" on a related functionality. So, I ignore the first person when actually busy, and attend to the second person right away always.

Also, there are people change the status to 'busy' when they are not actually available, extended lunches and what-not. And there are more people who forget to reset their status to 'available'. So setting status to 'busy' is no guarantee that you will not get a ping. Or a phone-call.

I still don't understand why you'd prefer a "hello" message in that context. If you're able to take quick questions but are working on something else, isn't it better to get the question in the initial message so you can decide if it's quick or deferable with minimal back and forth?