There are other issues. For example, Coinbase doesn't support it fully. See this thread for more info: https://twitter.com/coinfabrik/status/661992065841123328 but other wallet services like Coinkite does: http://blog.coinfabrik.com/independently-generating-your-pri...
If I want to spend coins from a multisig address, I create a transaction, sign it with my key and send it to the other party who can sign it and relay it or send it back to me and I can relay it. It's a very simple protocol.
Beyond this, there is a bigger reason: if you want a risk service like Sig3 or CryptoCorp you should adapt the system to every multisig wallet while if you have an standarized way to communicate you wrote an standard system.
