I agree with MIE that it also can be distracting (although I just try to lock my mind (and ears) at the show). I also had people bugging me just before the test if I have a test book they can read. Nope, I don't - it's in my head.
The eventing trainer I know in person showing different horses for her clients (I've seen her name 4 - 5 times on schedule list) basically reads patterns between the tests.
It's my first year showing BTW (and I did ride 2 tests at the show, and do ride 2-3 tests in my own ring on both of my horses
). Yes, nerves kick in - no question on that, but I still think using own memory is a better way to go.
BTW, I didn't know readers are not allowed for eventers! And I have to say not too many people showing dressage levels used readers at those shows I went to. I'd say reader was more of an exception than the rule.