Singular they is bad, because

Singular they is confusing

Even adopters of it will admit it is trivial to construct a sentence in which it is confusing to determine to whom the pronoum "they" refers.

Alice was having fun with Julia and John, but they were late to an appointment and couldn't stay for supper.

English is already full of omnibus words. We don't want to transform a pronoun in one.

Singular they simply isn't worth the trouble

I have never, ever found a single person (in the Internet or not) who took offense or complained when someone mistakingly picked {he,she} to refer to that person when the latter found {she,he} to be the "right" one. That's because:

  1. Of people we see, it is generally easy to use the right pronoun, because generally people conform to gender patterns, intrinsically and or extrinsically.
  2. Of people on the Internet, you have just to make a blind guess, and it's hard to find someone childish enough who would demand clairvoyance from his interlocutor.
  3. Although important for individuals, gender rarely matters to conversations.
  4. People assume good faith, and don't get triggered when others take a wrong guess for something that very often doesn't even matter. Of course, purposely using the wrong gender for a person is evil.

Defaulting to a gender does not imply misogyny or misanthropy

It is simply an intelligent decision. Use your brain power to do things that actually matter rather than alternating between two pronouns to be "fair".

Pronouns won't deliver equal rights

Women's rights obviously ought to be equal to men's. This is simply logical, no one in his right mind will dispute it.

You don't do that by fighting over pronouns though, but by supporting and pressing for laws, regulations and behaviors that promote such rights.

Using "they" when the gender of the subject cannot be ascertained benefits no one.

Don't go for red herrings.

A related, awesomely enlightening reading related to the subject: Is the term "man hours" appropriate for the work place?.