"them" is so much better than the alternatives for when you don't know a person's gender.
"Him"? 50% chance of being wrong and sounding stupid.
"Him or her"? 3x as many syllables.
"It"? Sounds like talking about an object, and when talking about someone you don't know the gender of, you're often talking about objects in the same sentence too. So "them" specifies the person, and "it" specifies the object.
"the [noun]"? Often requires more syllables, sometimes many more.
For example...
"[...] Always print the receipt and hand it to them directly."
We know that "it" is the receipt, and "them" refers to the customer/person. Any alternative would be wrong/awkward or too many syllables.
The "they" thing makes a lot of sense. I write a fair bit of documentation, and I make a point of referring to generic people/roles without gendered pronouns. If I'm talking about "a user", I'll refer to them as "them" or "the user."
At this point, I'm not sure what the alternatives are. Assuming gender seems very 1960s. Assigning a name (like "Alice" or "Bob") complicates the text too much.