Exactly why it's meant to be used with TFM/TDeint. NNEDI is used to generate the pixels sampled from by TDeint. I agree with you. NNEDI should not be used alone to deinterlace, but Yadif isn't flawless either. The way that NNEDI is used in standalone can introduce "stair stepping" aka aliasing. Which is why I use it as a sample platform, not final output.
However yadif has it's flaws as well, which mostly appear in the thresholds of edges:
