If you're designing the object, a shorter iteration process means faster learning and better creative flow. There's a significant difference between being able to think something up and try it out within an hour vs waiting a week for it. If you're a programmer, it's the difference between having a REPL and sending a deck of punch cards for overnight batch processing.

The current designs of single head FDM machines average about 60mm/s. The fastest XY heads can achieve 400mm/s, and the deltabots are around 300mm/s or so. There is an upper limit to how fast you can move a head around, but it doesn't feel like we've hit it yet.

In addition, I think as the FDM designs get explored and played out, we'll see explorations in other possibly faster methods. So as the home desktop printers get faster, I believe it'll find a niche that's different than what you'd do with online 3D printing services.