PCM requires no decoding at all. It is already decoded or unpacked. Any format you may have whether DD DTS TrueHD MasterAudio HiRes DD+ etc etc etc becomes PCM, usually at the receiver, but for a few in the disc player.
Why do we have these formats then? The main reason is the amount of space they save. However Dolby and DTS get paid for whenever their format is used. I said this at another forum recently: ...allowing more space means possibilities for further video decompression. I am glad to say that a little over half the BDs out are on the 50 GB discs. When doing preliminary research last year, I wasn't sure that PCM would cut it due to space restraints of the 25 GB discs. Seems like its not a big deal. However, I only have one 7.1 pcm track, but two 6.1 MA, and two 7.1 MA. (I support more channels!).