In the case of Apple, I'm pretty sure they aggressively ban anything that doesn't look like a real iDevice. I've heard the same of WhatsApp but I've successfully tested a couple of third party protocol implementations.
XMPP isn't great for mobile (bandwidth/battery usage) but I don't know enough to comment on that.
Have a browse back through some HN threads on chat and IM - these kinds of points come up every time. We've lost the war. One of my friends just asked me to install LINE and I'd be up to 6 different apps if I didn't just give up and go back to SMS.
Money quote: “XMPP is not suited for mobile devices. That’s a myth that has been around for ages. It is mostly spread by people who want to sell you their own proprietary instant messaging solution.”
From my own experience, high battery usage is the fault of the XMPP client. Regarding bandwith, I have been chatting over throttled 3G and regular 2G connections. The initial connection takes noticeably longer, otherwise everything except file transfers seems fine.
I work on a chat product that uses XMPP ....
There isn't, I don't think, a common protocol that could be used, is there?
I have successfully used this for voice calls between the chat clients Gajim/Pidgin (I do not remember which) and Google Talk (the now-discontinued XMPP client by Google). I have also done video calls between two Nokia N900 phones, to see if it works. Voice and video via XMPP has worked since approximately 2009 and is just neglected by the companies for business reasons.
I can join chatrooms in my XMPP client by joining a multi-user chat, like maemo%irc.freenode.net@irc.netlab.cz. It may seem useless at first, but I have found out that when on a train and using IRC directly, I timeout much more often than if using XMPP in the same situation.