I never bought the SIP is "simple" mantra. In fact, I was trying to counter it, certainly as far back as 2000, long before RFC3261 et al, with documents like my "Frequently Misunderstood Aspects of SIP Protocol" papers including a topic heading "The 'S' in SIP Does Not Stand For Simple" and rebuking the "SIP is HTTP" myth. You can find abbrevieted versions of that material here: http://www.toyz.org/cgi-bin/sipwiki.cgi/SipIntro

So if I am a SIP proponent, I'm not one that has suddenly changed my tune. I'll repeat what I said then, as to why I believed SIP would dominated the open VoIP protocols (which is certainly true now): "A side effect of SIP's IETF and Internet nature is that it is attractive to developers and designers because of its familiarity; it looks like an Internet protocol so developers can leverage previous experience and knowledge. In the end, SIP is probably not that remarkable on pure technical merit. And that's fine. It is probably mostly a matter of SIP being in the right place at the right time, and, as such, it has tremendous potential to be a very disruptive technology."