There is no reason to use any of them, I do not. Some like the affects that the functions provide, others do not and would rather stay as close to the original audio as possible. You really need to try each them and see if you like the resulting affect on the audio; you may love it or you may hate it.
It really depends on many things including personal preferences, and in many cases, a user's speaker array. For example, it your speakers for channels 6 & 7 are below par as compared to your first five channels, matrixing a 5.1 audio source to 7.1 can really mess-up the sound (or have no audible affect); I can raise my hand on this specific issue.
