Dolby Digital and DTS are necessary when your bandwidth is constrained to 2 channel PCM.
With 8 channel PCM you have theoretically bandwidth up to 36,900kb/s instead of Dolby Digital or DTS which are in the ball park of 640kb/s.
For example on blu ray discs if you only have optical out (max bandwidth 2 channel PCM) the default track is 640kb/s (and on DVD 448kb/s due to less available space), whereas the typical TrueHD track is 2,000-3,000kb/s and uncompressed Linear PCM tracks are in the 8,000 kb/s range. This increases dynamic range considerably and has an amazing effect on action movies...provided you have a robost HT setup.
That said, for those who are counting, 5,600KB/s 8-channel is the only equivalent of CD quality (which is already considered limited for music), so at this point in time home movie sound is not really where the industry wants to be.