Neat -- I'm still on OS X 10.6 and didn't know about this. Thanks for the pointer!
I guess the advantage of a dedicated pain box is that any device, regardless of operating system, could use it. Network Link Conditioner (and the two pieces of software mentioned so far by siblings of your comment) are all OS X-specific. (That's another webdev gripe for another day.)
Speaking to OS X specifically, it's dead-easy to set a machine up to route traffic, so you can use the Mac as a pain box, in this sense. Of course, the same largely applies to any OS these days.
I guess the advantage of a dedicated pain box is that any device, regardless of operating system, could use it. Network Link Conditioner (and the two pieces of software mentioned so far by siblings of your comment) are all OS X-specific. (That's another webdev gripe for another day.)