But the IME/PSP is intentionally and officially implementing an architecture which ensures that the manufacturer has ultimate control on the platform, and can run any code it pleases anytime on your computer. It runs at the deepest level (below OS, BIOS, VTd, SMM), and has maximum privileges on the platform. It runs all the time, so even as you have your computer switched off.
Have a look at Intel Anti Theft Technology for example.
http://www.intel.de/content/dam/doc/product-brief/mobile-com...
It utilizes the IME. It shows that the IME is able to completely take control away from you. It can be triggered while the computer is switched off by sending it a specific packet over 3G network. And while activated you cannot switch it on anymore and it does whatever it pleases, like continuously sending location data to Intel servers across whatever network it manages to get hold of. Nothing you could do about it.
Less spectacular is the problem that CoreBoot/LibreBoot are facing. It is not possible to install the firmware you wish, because the IME is more powerful than you on the platform and does not allow you to do so.
So you have a second computer sitting inside your computer which has full access to your resources and the manufacturer is controlling what it is doing.
So while we were maybe speculating about trusting the CPU manufacturer before, now we have no choice anymore. We have to trust him, he is the boss on the platform.