At the end of 2007, Charles (my secret brother^^) built the murderbox 001 and if you remember the days before Charles made this marvelous system - it changed the aesthetics of "modding" (a word which we don't like to use for such systems) a lot. Before there was mostly "bling" added to systems and the designs were rather "cartoonish". I love all designs though, no matter how it looks, as long as the builder had a vision and executed towards this vision.
He sleeved his 24-pin, because the standard PSU cables of the 24-pin were a visual distraction from the rest of the design. So he tried to get a subtle and uniform look to the chaotic 24-pin wires. The solution is what changed the cable-designs a lot. See the system here: http://www.million-dollar-pc.com/sys.../murderbox.htm
I also have to laugh, when I read product descriptions of "modding-shops", when they refer to "it enhances your cooling, airflow ... and brings you more girls". To Charles and me, it was always a different perspective: Design and aesthetics of the most annoying thing in a system ... little wires which never had design in mind and were too thin to create controlled "shapes and lines". Cable sleeving is of course not needed for anyone who wants function and performance. Only fools and stupid shop guys would tell you about "technical advantages". You all know these fool-shop-owners who do everything to sell their items, because they are so desperate to earn their bucks.
So ultimately and personally I say: It's an option to perfectionize designs if cables are visible (of course not with extensions, where you have to hide the original-cable-clutter somewhere ... which is double nonsense for people who want to have form and function combined). I actually see design of cables as something, where you use cables to create flow through lines and shapes. To add color, where color is needed ... I see computers like a 3D painting and cables allow you to use something which is technically needed to create "the strokes". With other items, you don't have the freedom to work "3D", because you can't just take your GPU and bend it ... or other hardware.