I'm a birds nest man myself. My needs are constantly changing so I just leave them in a giant tangle on my desk. It may not be aesthetically pleasing but its extraordinarily functional-I can grab any cable at any time, and making room for another project is as simple as shoving the pile of cords into a denser pile.

Here, here.

Cable management is essential for a data center where multiple sets of maintenance hands, multiple owners, SLAs, fire codes, and cooling/venting needs demand organized layouts.

Cable management is also important for tight spaces (inside walls) and extremely low-duty-cycle, but mission-critical wiring such as a home's breaker-box.

Is it necessary for the typical modern desktop with a few AC Power, networking, and USB cords?

Um, NO.

I generally agree that it doesn't matter a lot, but I've found one case where having well-arranged cables is useful: if you want to move the equipment a short distance temporarily (for example, moving a tower case out from between/inside furniture, or turning a monitor around to show it to someone else). Having the cables to the device arranged so they all have sufficient slack, and are not tangled with each other, means you can do this without having to unplug anything (and possibly shut down/lose use of the equipment).

Ikea cable trays under the desk: http://www.ikea.com/us/en/catalog/products/50035115

And on the desk I use some plastic tubes that hold several cables together, also from Ikea.