When playing everyone usually has drink of some sort so they are always used.
They are at the side so don't get in the way and also help delineate seating. Pet hate of mine is feeling crowded at a poker table so when it came to building my own I went for 8 (max) clearly marked seats - with the cup holders pretty much forcing players to not encroach.
I didn't like the idea of them in the middle - then they definitely would be in the way imo, jabbing into my stomach. It's not as though you have to move to pick up a drink with them to one side.
I can just take them off but there's no reason to unless I'm completely collapsing the table (which I can do).
They're also small enough, and at such a height, as to not bash into thighs when getting up/sitting down.
I do have some bigger ones that are more like side-tables than cup holders but I haven't used them yet as I think they would get in the way a bit.