Okay, I know... "Security".
But that's like saying, "Mommy says so." It doesn't really answer the question. What specifically are you securing? It seems to me that the only thing I could expose or compromise is my own Confluence site.
Can I get a bit more detail? We _really_ need this macro. The iFrame macro does not resize (or several other things) with the content and is a truely suboptimal workaround.