I don't quite understand how not to memorise...don't they just kind of seep in over time? I tend to find after I've played a passage 10, 20, 30 times, then I can play from memory without having to make an effort to memorise.
It seeps in, but won't be secure. If one has to stop the the middle or have a memory lapse, it is likely that one will be unable to resume.
It doesn't even seep in, for me. I can't imagine how many times I must have played my Grade 8 pieces when preparing for the exam, but it must have been into three digits. I tried (out of interest) playing one of them entirely from memory, and managed about a bar and a half. I don't think I'm actually reading the piece when I play it, but I absolutely have to have the page in front of me or I can't play it. It may even be that it acts as some kind of visual prompt to trigger the memory. Who knows?