666 isn't used in the USA much. A large percentage of the mobile phone providers in the US do not use GSM as their technology, but use CDMA (I believe) and these other technologies do not use SIM cards. So built in phone or the bluetooth profile for using the phone's SIM card would be useless to a large number of mobile phone users in the US.

I want to pick my phone carrier based on the signal availability where I normally drive around, and not on the technology they use to implement their network and if it works with my car, so the full hands free profile bluetooth prep is the one I went with. I'd LOVE to have gone with the SIM card 666 version.... unfortunately I live in the USA, where that would restrict me.