Arguably, part of this comes down to the same argument about "Roll your own" vs. using an appliance
All equipment fails eventually. If you built the system and it fails, it's your problem. If you buy a system from the vendor, and it fails, it's their problem.
With good support, you have trained people ready to back you up. Companies like Cisco, Juniper, NetApp, etc. are successful because they provide quality products backed with quality support. When they fail (and sometimes they do), their business is harmed.
High end equipment can come with a good support contract. If the firewall crashes at 3AM on the Saturday after New Years Eve, I can get a Vendor technician on the phone in 5 minutes. A technician can be on site in 2 hours and swap out the failed component for me. If the router supports a large business where downtime can cause expensive losses, then it might be worth it to get a high end router. $10,000 or $100,000 doesn't seem that expensive when it's supporting a $20-million or $200-million business, where downtime can cost the company thousands of dollars per hour.
In many cases these high end routers are too expensive or are unnecessary, or you can't get a high end router due to budgetary or political reasons. Sometimes, a custom pizza box or a Soekris box is more appropriate.