One benefit of swapping, at least in my experience, is longer times between needing to pay for hydro tests. The airgas I swap at is sure to give me a tank with the same or earlier date.
You swap at every fill? I only swap when I need a retest. Otherwise, they fill the tanks I bring in.
They were out of steel 20s last year or so when I went in and I got the bonus of swapping to an aluminum tank. Which is nice with the larger tanks.