It's pretty much like having a hidden safe: Before anyone can even try to break in to it, they have to find it.
Having a hidden safe and telling people you have a hidden safe filled with goodies inside your house defeats the point of having a hidden safe. The primary 'obfuscation' in this case is not letting people know you even have one.
However letting people know you have a safe worth looking for, and a narrow search zone - your house - to find it in has just blown 80% of the security - obfuscation - you are depending on.
Letting people know this secure processor exists is the same as letting people know you have a hidden safe. At this point you can no longer rely on obfuscation, you have to rely on the strength of the security - quality of the manufacture, strength of the walls, hinges, door, locking mechanism, unlocking mechanism. Therefore once the cat is out of the bag about the secure processor, there will be people actively trying to break it, therefore you now must rely on the strength of the security on the processor - no bugs in its firmware, no programmatic attack vectors from the main processors or I/O (can you access it via the USB port? If it's firmware is upgrade-able there must be some I/O channel that has access to it).
As a poster above stated, look how well relying on obfuscation - once it was known such a thing existed - worked for Intel.