I've always (or, for many years) wondered why this wasn't more common. Every network I've ever built has been routed by Linux or a BSD, though I've maintained a few with Cisco routers. Partly because it's what I know, and partly because there's never enough in the budget to to everything the client and I wanted to do with Cisco or even Juniper gear.
Now, admittedly, I've never worked on big networks. But, I know what I like, and I know working on a Cisco feels like 1996, and that working on a full-blown modern UNIX/Linux system feels like home. And, fast network gear is available for Linux and the BSDs. 10Gb interfaces that work with Linux/BSD are not difficult to find, and 1Gb interfaces (sometimes several of them) have been standard on every server and barebones rackmount on the market for a decade.
I understand there are special purpose boxes that do things no off-the-shelf commodity box can do (I don't know enough about high end networking to know where to draw that line), but I honestly doubt 95% of business networks need them. And, yet, a huge percentage of business networks that have real routers (rather than little wifi routers from LinkSys/NetGear or whatever) have a Cisco of some sort, even when they don't have anyone on staff who knows how to work the thing. I've walked into several businesses that had expertise in house for everything except their router, which is also weird...why pick a Cisco router for your little ~100-1000 node network, if you've got Linux or BSD guys hanging out in the server room every day? They'd call someone and sit around waiting when their network was acting up.
I learned how to use Cisco routers just so I'd be able to replace them easily (enough knowledge to read the routing tables, firewall rules, etc., but not enough to do much useful), when I found situations like that.