I found most code editors have a "Embed on your page option", I think it would be great if you could embed
<iframes> in answers and questions.
I don't think this is a good idea.
<iframe>'s in most cases load external content and run into the similar problem link only answers do. The external content can go dead, and the content is lost forever. Why not just bring that content into the Stack Exchange ecosystem instead of hosting it elsewhere?
Not to mention the issues with loading external resources over SSL. And as iStimple mentions, Stack Exchange can't control what is being loaded via that iframe (think drive by downloads, or alert popups).