Premultiplied alpha is also more "correct" in that it separates how much each pixel covers things behind it (the alpha value) from the amount of light it is ref

Premultiplied alpha is also more "correct" in that it separates how much each pixel covers things behind it (the alpha value) from the amount of light it is reflecting or emitting (the color values). These two values should really be interpolated separately, and that's what premultiplied alpha gives you.