This stops the possibility of making iframe based browsers.
What do you have to lose by allowing iframes to your site?
Maybe a more secure alternative to iframes can emerge. Something like Chrome's WebViews.
Even though as of right now native browsers can intercept your data as well, it's a matter of trusting the tool.
Example: http://www.jayssite.com/misc/iframesample.html (not my site, I googled it)
