My house is 30 years old and has been painted at least 4 times, the last being 7 years ago. I have never used a primer on the masonite siding but see lots of articles saying you should use a primer. Since the body of the house has previously been painted and is in good shape (no cracks, chalking etc.) do you still recommend a primer for the body? I have primed all of the trim at this point.
Sound like your siding is is good shape, overall. A primer isn't necessary in your situation. If you have any areas that are peeling, then you will need to spot prime these areas before painting.
Pay special attention to the lip of the siding. This area is masonite sidings weak spot.