The Mini isn't just a consumer product; it's also Apple's only server. And as has been revealed, companies developing iOS apps oftentimes have hundreds of racked Minis, such as Facebook, although the exact numbers and how many companies do this is unknown, but it's feasible this use case of the Mini is not insubstantial. The Mini is oddly enough a decent server because of it's form factor relative to all other Mac options which use up much more space.
In other words, the Server Mini and the Mac Pro do not actually overlap markets; the new Mac Pro is more about rendering than networking, and isn't actually competitive from a compute/dollar standpoint, nor a size standpoint. Four $1200 Minis (the inevitable high end i7 option) will likely have significantly more compute resources in total (something like 14,000 x 4 on Geekbench, around 56,000 total) compared to one Mac Pro (over 33,000).