I've been looking at the 2A03 TEST pin and it looks like it'd be really useful for APU test ROMs. It would allow more aspects to be automatically tested by a ROM, rather than requiring listening tests. This would simplify automatic validation suites in emulators, though listening tests only need to be done once, then the audio recording can be compared for binary equality. The only problem is that people couldn't verify the test ROMs on hardware without enabling the test mode on their NES.