<< Dolby Digital EX needs 2 centers >>
Actually, that is incorrect. In Dolby EX, the rear center channel is matrixed, meaning that it will extract information from the rear surrounds to create a phantom channel. Dolby EX is still a 5.1 discrete technology; in fact, theaters with Dolby Digital systems can easily upgrade to EX by adding the Dolby EX adapter to their decoding equipment (and you can even do this at home without buying a new receiver/preamp if you have preouts for rear speakers; just run the rear preouts to an old Pro Logic decoder in Dolby-3 mode). Right now as it stands, a discrete version of EX does not exist.
DTS-ES Discrete on the other hand actually does make use of a rear center speaker, because there's actually a discrete 6th channel encoded in the surround mix. Movies like Gladiator have ES Discrete soundmixes. A "matrix" variety of ES also exists, which behaves similar to Dolby EX.