I think it depends on your skin type. I worship my Smashbox Photo Finish primer because of my intensely oily skin- my make up used to always fade and melt off. Also, do you live in a hot and humid area? Primer's are a huge help in keeping your make up on.
As for MMU... The first time I tried it, I tried it without primer. BIG mistake! Looked cakey and awful. Primers will soften your face, hide pores, and give a nice, matte base for your MMU.