If you don't get support for that subscription, I can't see a difference worth $350 between RHEL self-support subscription and CentOS.
EDIT: ate a word
EDIT 2: self-support desktop subscription is just $49. But the point is the same: no support.
* Access to the RHEL repositories, with faster updates than CentOS or alternatives.
* Ability to upgrade to supported tier.
* Possibly: access to RHN pages with their forums, documentation, etc?
None of which would be huge advantages to me, but I can totally see the potential value.