At OSCON, I spoke about NUnit for Cross-Platform development. Here are some screenshots from that talk, showing NUnit running under Mono 1.1.13 on Ubuntu Linux. (Click to enlarge)
People want what they want. If NUnit doesn't have a feature that they need, they ask for it. Of course, we can't implement all of them, so some people want to be able to create there own test runner using the NUnit assemblies.