If you look at the Salmple, it has four discrete voice outputs in addition to the mix out. By using the 4-in stereo mixer with it, you can have more control over the Salmple's sound...sources can be mixed the way you want, and panned in ways that give the Salmple an actual STEREO output instead of the single mono "mix".
-- Lugia
Yeah I can see that. Think I was just confusing myself with the whole "drums should be mono" statement, but that makes sense now as you're referring to the audio path re panning, etc. Looks like the A-138s shall be added to the wishlist then :)