According to Facebook documentation, FB.XFBML.parse is described as
This function parses and renders XFBML markup in a document on the fly. This could be used if you send XFBML from your server via ajax and want to render it client side. XFBML enables you to incorporate FBML into your websites and IFrame applications.
However, while Facebook has provided a HTML5 implementation for that, I think this function should be renamed to better reflect its functionality.