Counterparty was originally designed to use the OP_RETURN output to store all of its message data, which I feel is very elegant, and leaves a minimal impact on the blockchain.
Message data does not belong in the blockchain,
only financial transactions.
If by "message data" you mean something that is in fact financial transaction itself, then please write a draft BIP explaining why you think more than 40 bytes OP_RETURN space is needed.
We planned all of our message formats around the 80 byte limit announced by Gavin on the official Bitcoin blog.
There is no official Bitcoin blog, or official Bitcoin anything.
Gavin does not speak for Bitcoin. Jeff does not speak for Bitcoin. I do not speak for Bitcoin.
We only use multi-sig outputs because we have no other choice. We don't want to extend the Bitcoin protocol. We want to do something entirely within it, and as simply and directly as possible, for the benefits to stability, security etc..
The Bitcoin protocol, today, does not support data storage, nor what (I presume) Counterparty wants to do (otherwise it would just be Bitcoin, not Counterparty!).
So, if you want to do more than what that Bitcoin protocol does, you must extend Bitcoin.
That extending should be done in a rational, open manner, using the BIP process.
I'm just going to ignore the trolls who demand that because they want to change Bitcoin, I must stop using it.