Support for <iframe> is still there in HTML 5, so I don’t think this will change in the near future.
To answer your other questions:
<iframe>s (as frames in general) are most of the time not user-friendly:- They don’t allow easy access of the content in the frame via an URL (without losing the content outside of the frame at least).
- Most “technophobe” users are irritated by frames.
- As far as I know they are slower to render for browsers
- Alternatives include dynamic page generation (SSI, PHP, Rails and so on) and using JavaScript / AJAX to change contents of e.g. a
<div>
To be clear: I’m talking about <iframe> as an interface element. Not a hidden element for loading other stuff like e.g. Google Mail does.