The most frustrating thing I find as a teacher of A level Maths is that students seem so reluctant to learn trigonometric identities and ratios, e.g. sin(30)=1/2.
Why is this?
Having a really thorough knowledge of "facts" such as these makes questions so much easier. Students often complain about the difficulty of proving trigonometric identities in C3 questions. I'm not surprised by this because they don't have the standard results at their fingertips. It's no use saying "they're in the formula book" because many of them aren't and, even if they were, you wouldn't necessarily be inspired to look for the right thing. For example, if I see sin(x)cos(x) I immediately think of 1/2 sin(2x) Even if the formula for sin(2x) was in the formula book, I don't think you'd make that connection if you hadn't learned it.
Similarly for learning trigonometric values of standard angles. No-one would seriously argue (would they?) that you don't need to learn your times tables, so why be so stubborn about learning trig values? It's true that some calculators give exact answers for these so you don't "need" to learn them, but why spend time faffing about with the calculator when you could be getting on with the question. Calculators can tell you what 6 x 4 is, but it's so much easier if the answer 24 immediately jumps into your head. Isn't it?