Black dead pixels cost nothing, but dead subpixels, or pixels stuck to displaying colour are very bad.

Even if adding more resolution makes you unable to resolve the pixels, you can still always see light, in the same way you have no hope to resolve stars, but you can clearly see their light.

Luckily once we get oled, it no longer matters, because there is no backlight and you can always set misbehaving pixels to black.