The way you send it to another party is not standarized. Think in your grandpa, he would want to just create a transaction while everything else happens behind the scene. Sending an email is a manual process and you need to recheck if the other party received it.
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.
You can send an email(messaging) behind the scene, need not be manual nor is a task that require standardization.
Sig3 is more secure theoretically but its an overkill & non-requirement nevertheless. Why not have Sig4? Sig100 would be theoretically even more secure!
> You can send an email(messaging) behind the scene
So... how Coinbase handles it, what is the format? Coinkite? Copay? The nth popular wallet? You can easily see that you need a standarization, it doesn't matter if its by email, morse, or slack.
Now, Sig3 is the name of a service, with Sig3 you can use any n-m wallet.
