Normally, I’m willing to join the singular “they” love-fest as much as the next linguist or descriptivist. However, there is one context I noticed recently when singular “they” is actually a really unhelpful term to use.
So let’s say you’re doing some fieldwork, and let’s say the language you’re working on has person and number marking on the verb, but no gender. And let’s say you’re trying to get a few verb paradigms, and your language consultant, like most speakers of English, has a robust singular “they” construction. Then you might end up with a conversation like this:
Linguist: So, how would you say “he’s thinking”?
Linguist: And would this also mean “she’s thinking”?
Speaker: Sure, he’s thinking, she’s thinking, it just means they’re thinking.
Linguist: Ok, so if you have a couple people, and they’re all thinking, you would also say adahntehiloʔa?
Speaker: Oh, no, you’d have to change it. Adahntehiloʔa can only mean that one person is thinking.
Using singular “they” is a very reasonable strategy here to get around the fact that many languages don’t make the same gender distinction as English does in the third person singular, so it’s awkward to be obliged to specify a gender distinction in translation. And in a lot of real-life situations this would probably be fine. However, this creates some unfortunate ambiguities when you’re really going into the details, especially when you haven’t worked with the language long enough to have a good recognition of the affixes.
So this is one circumstance in which I generally use s/he or try to alternate between using one gendered pronoun for a while and then the other, but never singular “they”. On the other hand, I use “you guys”/”you all”/etc all the time in elicitation, because most languages do make a distinction between singular and plural second persons.