No more so than a proprietary protocol. All the IM clients and apps are different, just like all the email clients.

What matters is that I should be able to just get anybody's IM address and message them with my client and service of choice.

What? No, if my colleagues and I all decide to use ${CLOSED_CHAT_SERVICE}, the user experience is generally going to be consistent.

> What matters is that I should be able to just get anybody's IM address and message them with my client and service of choice.

This is what matters to you. I don't care what client I use if I can't use it in a way that meets my business needs, like sharing screen shots.

But even in email, if a group of people decide to use only Gmail web interface for example, the user experience is going to be consistent for them as well.

The underlying detail about email being an open protocol will be transparent to them.

In fact, an email client could be designed to implement instant messaging — by sending messages as emails — and if two people both use that same client, the experience would be indistinguishable from an IM service!

You'd still have a conversation history, you'd still have offline delivery, you'd still be able to send graphics and animations and audio attachments and anything else that HTML can render.

(I get that current email protocols may not be lightweight/efficient enough for rapid delivery of a large amount of short messages though.)