It also has malicious uses, you could embed external malicious links that will be processed on the user side without their knowledge. Also leaving an open iframe tag anywhere on the page will result in it not being displayed properly.

The iframe tag doesn't get parsed when it is used as a content of a post, it will be parsed as standard text, however, should the iframe tag be used in a signature, it will be parsed as an iframe tag. There is no option for us to be able to allow a certain word on the posts, and not allow it on the signature.

Sorry for the inconvinience, you could use [iframe] or something instead for discussion purposes.