When you're spending $3500 on a maxed out 15" rMBP config, and $1200+ on a 30" monitor ($4700 total), $350 is still a lot, but not that huge of a stretch, and a base model Mac Mini just isn't going to cut it, especially if you do most of your work on the rMBP and don't want to worry about managing multiple machines and want a seamless experience.
Let's say you already having the 30" monitor, buying this to add TB capabilities instead of buying a $999 Apple Thunderbolt Display (which lacks audio out and lacks USB 3.0) may actually make sense.
Personally I wouldn't buy at this price point, but I can imagine someone who would, or I can imagine companies buying their developers or other professional employees this.