I hear that a lot, but I kind of have to wonder - the singular "they" has been part of the English language since the late 15th century. How many hundreds of years does it need to be in use before it isn't considered against the rules?
Well, nobody said grammar Nazis were rational.

I agree with you in principle, but in practice I have a very hard time forcing myself to adopt the singular "they," and I know a lot of people would think less of my writing if I did. Sad but true. The question then becomes, is this particular linguistic hill worth dying on?