I actually learned to fly doing the pre-flight, post flight, and all the cockpit checks from memory, and I find that I had a better understanding of why we do certain checks,when they need to be done and once you are proficient in that type I never missed any checks, unlike my students who have to do EVERYTHING from the checklist and cannot think outside of that realm when flying...if I took their checklist away they wouldn't know what to look for on pre-flight or what to do before/cruise/after landing as they are not having to think on their own....
Now when operating complex or two crew machines they are valuable aids when flying...
Fatigue.