Most users I've come across that use the texting of their phones also have a data plan. Case in point: every iPhone user that is forced to pay for a data plan simply because they own an iPhone. Additionally, most users under-utilize their data plans (particularly those grandfathered under the old "unlimited" data plans), such that the minimal extra data as a result of XMPP messages being passed to your phone would be negligible given your total data capacity (even assuming that the "always on" application would have to poll for messages from the server inefficiently, which it wouldn't).

Second, I'm not sure you understand push notifications. Maybe I'm not sure I understand push notifications. But my logic follows thusly: for a year or so, I had an iPhone 3G, which theoretically could not "background" its applications, or have apps that were running constantly other than those that were built-in or somehow tied to underlying system processes. And yet, I was able to receive push notifications for applications that weren't running (see: Words with Friends). The notifications were "pushed" to my phone on behalf of the servers behind the applications I had installed, in much the same if not the exact way that a text message is received by one's phone by way of control messages from the connected tower. That means that push notifications do not require running applications, no more so than text messages require you to be constantly polling the towers to ask "Do I have any new messages?"

The messages are delivered on an as-needed basis, and are re-transmitted or re-issued in the event of network failure or having your phone turned off. The only reason anybody recommends disabling push notifications is because it requires connecting to something more sophisticated than the Edge network, and HSPA ("3G") inherently takes more battery life than the Edge protocols.

I don't know what kind of crazy applications you're running on your phone that drag down your battery so, but I would venture that a simple IM client,ran in the background (or not at all, if you were only responding to IMs as a result of push notifications) would be fairly battery friendly.

And fine, Facebook and Twitter allow you to transmit status to people. I've expressed my distaste for these services before. And the convention for IM status is different than those for Facebook or Twitter, in that the statuses are not "stored" anywhere, they are simply a temporary state assigned to a user. That seems a much more logical application of a "status" when applied to communication like texting or IM.

It may be justified that they charge for texting, but their margins are incredible, especially when you consider overages. I think the featureset surrounding IM improves upon texting, is based on a better underlying technology, and can be considerably more "open" if you decide not to depend on free-ish corporate services.