O.K Example:
[3/7/2010 12:10 PM] Detected video or audio codec: V_MPEG4/ISO/AVC.
[3/7/2010 12:10 PM] Video dimensions (width x height) are 1920 x 1072.
[3/7/2010 12:10 PM] [W]C:\Users\Will\Desktop\XboX\XenonMKV\tools\mediainfo\mediainfo.exe "D:\XBOX\part-001.mkv"
[3/7/2010 12:10 PM] Video display aspect ratio (DAR) is 16/9.
[3/7/2010 12:10 PM] Rounding up the pixel aspect ratio from 953:960 to 1:1.
[3/7/2010 12:10 PM] Video pixel aspect ratio (PAR) is 1:1.
See:
http://en.wikipedia.org/wiki/Pixel_aspect_ratioIn terms of picture quality, the optimum pixel aspect ratio for any given picture dimension is 1:1.
Any other pixel aspect ratio will cause the number of pixels in one direction (either horizontally or vertically) to be reduced and thus image will lose resolution and will contain less perceived detail.
So that is basically why it's enabled by default and I recommend you leave it on too.