This contradicts my experience - I've repeatedly witnessed only a few mb/sec of 64-byte packets making software-based routers fall over, including just last month.
It's easy to get 6Mpps using Vyatta or most other software based routers on modern server grade hardware. If both your link and hardware capacity don't exceed the maximum load the bad guys can throw at you, then you are doomed to failure regardless without upstream cooperation. In the case of an enterprise, neither of those two things are always guaranteed. Software based routers work great for many use cases, it just depends on what you are looking for. And sometimes they are the only option, for example as services migrate into the cloud.
Software based routers are only going to get better. Roland, if you are in the Bay Area I invite you to drop by Vyatta HQ in Belmont and I will show you a demo of our next generation software which is doing an 5x increase in throughput over our last generation on the same off the shelf Intel X86 processors. We expect that number to only increase over the short term. Lot's of CPU horsepower solves a multitude of woes.
The one thing I do agree with you on is…
One can and should test the specific performance envelope of any prospective infrastructure purchase, of course.