Not that it answers the question, but the wikipedia article on the topic has an interesting point about the strangeness of this use of 'they' being semantically singular but syntactically plural. What this means is that the singular 'they' refers to one person out in the real world, but as far as grammar is concerned, it behaves like it were plural. Specifically the verb whose subject it is is still conjugated for a plural subject. Nobody would say
*I met someone on the internet yesterday. They has an interesting blogbut rather
I met someone on the internet yesterday. They have an interesting blog
The interesting point it makes is this: it's not a strong argument against using 'they' in this way to plead that it makes the language hopelessly complicated and weird in the face of this inconsistency. Maybe it is more complicated and weird, but we've already done this once. The word "you", which is syntactically plural, (if I've got the story right, someone yell at me if don't) used to be semantically plural, but got used to respectfully denote a second-person singular referent, and from there elbowed the familiar second-person-singular 'thou' right off the table. So we're left with "you have" instead of "thou hast", and somehow we get by.