Even if you had a big enough pipe the latency would kill you. If you've ever played wow you know that it is very frustrating when latency alters the position of your colleagues or the monsters. WoW tried to cope with that by handling most calculations (for fights etc.) locally, and syncing with the server. That way it doesn't matter that it takes 100ms to talk to the server, your 0.5s cooldown isn't actually delayed at all. With rendering though it would take away all that local benefit.

WoW doesn't even really hammer good current graphics cards, and the style of graphics doesn't really need it anyway (I remember killing Vaelastrasz in the original WoW on my old computer as the main tank with 3fps - but my newer cards all do 60fps without a problem on every fight).

This is all about rendering movies or cut sequences for games. Basically pre-rendering. Presumably playing it back to a single box is so that the designer/editor can actually see what they are doing in near realtime. This is a big goal of modern movie editors. Peter Jackson for Lord of the Rings got a big enough render-farm that he could review changes overnight for sequences they were working on. Doing that with a few seconds latency but effectively in realtime would be a big benefit.