Image composition and filtering were what did it for me.
Composition was in particular key for me. If you're ever doing layered composition, rendering to a transparent texture first, then pre-multiplied is essential to get the right result.
Some other useful blend modes are not possible (shy of shaders) without pre-multiplied data as well - Multiply in particular.
Just realised that one of the posts I read a while back that I found useful was the TomF one in the OP's post - just his link was broken by the forum!
As you point at additive blending for everything, how exactly do you subtract color from the framebuffer, like you do with regular blending...?
What exactly do you mean by subtract? Actually, I think that question is the nub of why people don't get this - standard blending should be an additive / accumulative process, and there's a reason all blend modes (PorterDuff, etc.) are expressed in terms of pre-multiplied alpha. The blend function you give isn't an Add blend though - if you draw fully opaque black on white you'll end up with black (GL_ONE_MINUS_SRC_ALPHA on dst).