I can almost always remember every hole of every course I play for at least a few weeks, sometimes months or years. I think thats just the because of the way my memory works and my passion for golf architecture, where I am constantly studying/analyzing each hole as I play.
However, I do not believe every golf hole should be designed with the intent to be "memorable", just as there is no such thing as 18 signature holes, or even as some argue, signature holes in general. A golf course, like any work of art (painting, book, movie, etc) must have highs and lows, it cant just be non-stop action, there must be a sequencing that moves you throughout the property that makes the great holes really pop. Sure "transition holes" as they are sometimes called can be good and memorable as well, but without relative highs and lows I feel that everything would get jumbled together and the great holes that are really meant to stick out in ones memory wouldn't mean as much or have their desired impact.
Look at the routings of some courses with great dune/seaside properties like Sebonack, Friars Head, Cypress, etc... there is movement that takes you in and out that keeps you interested and wanting more of the great stuff.