My parish is very crowded, so the ushers, who function as greeters, are essential to getting everyone a seat. If you arrive ten minutes before Mass starts with your wife and two kids, good luck finding a pew with seats for four without them.
Many people are called to a lay vocation, but they still want to participate in other areas of the Mass than praying in the pew. Yes, both are valid, and in reality, everyone there who participates in spirit is participating as much as anyone else, but some people like it. I don’t see any reason to knock it.
Furthermore, if I were to visit an EF Mass, and I weren’t Catholic, I’d be completely shocked. I wouldn’t be able to understand anything. No one would explain anything to me. It’d look like a weird, creepy cult. Greeters could fix that and make people want to come back.
It’s not a useless position. Especially when you need them as ushers.