I think that specifying a psmf stream only determines what data scePsmf will return to you.  I don't think it actually affects the mpeg.  Could test this by creating an mpeg with two streams: a green solid image and a red one.  Then try specifying the psmf stream.

Also, concurrent mpegs (I've never seen this, but sometimes games play mpegs in small boxes, or in backgrounds, it's certainly not impossible that some game uses it.)

-[Unknown]