Except that iframes introduce additional http requests, difficulties with selecting text and all the other annoyances that come with them.
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First, you have a need. Then you get a crippled native "solution", that emerges from some committee and therefore doesn't really solve anything. In lieu of alternatives, a lot of people use it anyway. Then the problem is ignored long enough so other people come up with hacks to solve the same problem slightly less bad. Then the problem is ignored semi-permanently because 'hey, you can just use one of those hacks!'. (Look up iframe seamless attribute.)