Is it? Seems to me it's a way to decouple a high-end GPU from the base unit. Take me for example. I do some light video and audio editing from time to time, and I do a lot of Adobe CS work. Any gaming I do, I do on a console. So I do not need a beefy GPU at all, it would be a waste of my money to spec it in my Mac, and a more or less base model Mini with a decent amount of RAM would work just fine for my current tasks.
But let's say next year I take on a bunch of video work and I find my Mini's integrated graphics aren't up to the task. Do I buy and spend time configuring a whole new machine? No. I take my existing Mini and I plug an eEPU into it, just as easily as I'd plug in a Thunderbolt SSD for fast storage. Boom. My little base-model Mac is now a whole lot faster for this purpose. Hell, maybe I work in an office and we swap said eGPU around depending on who's got video to render that week. Now, tell me how that's insane or idiotic.