Unless you are setting up the arena (and even then it is in the USEF rule book) or taking a test, it is immaterial to know the 17 letters. Riding a test is easier learned by riding the patterns.
I'm not sure I understand this comment - I'm trying to imagine situations where I wouldn't have to lay out an arena, and there were only a few (lessons or clinics at a dressage barn or actually at a show) - I almost always had to lay out the arena.
Also, I learned and taught tests by drawing or walking the patterns, which required that I lay out the letters first, so mnemoics like these came in very handy. On rainy days I chalked the letters out in a rectangle in the barn aisle and had my students walk, trot and "canter" the patterns on foot - funny and an effective way to learn the tests.
I always advised and believe that it's a bad idea to drill the test on your horse to learn it as they learn to anticipate.
Finally, who's talking about 17 letters? You don't start seeing the additional 8 letters until Second Level.