Why are spaced repetition "practitioners" warning against that?
I have a "Roman Emperors" Anki deck. It serves absolutely no purpose except making me feel good about it, but it works well (except maybe for the final few emperors, those tend to be leeches).
If you're truly interested in Roman Emperors, that's fine, of course. The warning is more about "I just started spaced repetition, I need cards quick, so let's do capitals, US states, and other random trivia".
I admit birthdays of acquaintances don't quite fit under "random trivia", but I'll still maintain that learning random birthdays seems useless, when the calendar works so well.
I have a "Roman Emperors" Anki deck. It serves absolutely no purpose except making me feel good about it, but it works well (except maybe for the final few emperors, those tend to be leeches).