The main reason I do the mix is mainly because Premo is expensive for the amount you get. It's more cost effective for larger models with the Super Sculpey mixed in. I also find adding the Sculpey to it makes it a nicer material to work with, like the Premo is tough and needs to be worked to get malleable, and the Sculpey can be too much the other way, ie droopy. So together they make a sturdy easy to work with material.
Premo is also nice to if you wish to avoid painting and if you are working on really small stuff. Think those minature foods you see others make. I paint everything and work on the larger side so I've never had the reason to use just Premo. Though one could always just use a tin/bland sculpey base and make the final layer of Premo....I may try that one day.
I am still a beginner in sculpting but I really enjoy it. And I was quite desperate trying to sculpt precise things with my Fimo (wich is, I guess, more or less like the Premo) and could get my clay soft enough.