primer and paint as separate materials is always my preferred choice. My fav is Sherwin williams Preprite Problock.
Primers help with adhesion and can help cover challenging colors. If you are "repainting" in a color that is similar some simple cleaning prep may be all that is needed.
I agree with Kevin. I too do not use the combo primer and paint. This is a fine product if your colors are not all that different or the surface is in really good shape, But for any paint color changes or simply a surface that is less then ideal, I would prime using a good quality primer then use a good quality paint. Cost more, but the results both immediate and long term are worth it.
Paint/primerall in ones are basically just thicker coatings in most cases. If the surface needs priming, then use a separateprimer and then paint. Can't tell fromyour question, but if you are painting interior walls, you may not need aprimer at all for a same color. Just becareful about sheen and alkyd to latex changes (i.e. going from semi-gloss toflat would need a primer going from oil based to latex needs to be primed). If you are using eggshell/satin latex over eggshell/satinlatex then you most likely can just paint over the old surface with your newacrylic/latex paint.<?xml:namespace prefix = o />
Most of your better paints that have acrylic in them are a self priming paint and do not need a separate primer coat. I always use two coats however, just to make sure that you get enough mils on the wall to get a more scrubbable finish. Unless you are going over gloss or oil with flat you do not need a separate primer.