Offsite Ads are the fee sellers argue about most, partly because of how they work: Etsy advertises your listings on Google, social platforms and the rest, and if a buyer clicks one of those ads and purchases within the attribution window, you pay 12–15% of the order — 15% for shops under $10,000 in trailing-year revenue (who can opt out), 12% above it (where staying in is a condition of selling on Etsy). So "worth it?" is only a real question for smaller shops; bigger shops should ask "can my prices survive it?"
Your profit margin on an organic sale, and the offsite fee percentage. If your organic margin is 34% and the fee is 15%, an offsite-attributed sale leaves you 19% — thinner, but profitable: a sale you arguably wouldn't have had. If your organic margin is 12%, that same sale puts you 3% underwater, and "free extra sales" are costing you money.
Above roughly 30% true margin: leave them on (or accept them cheerfully) — every attributed sale is profit you didn't chase. Between 15% and 30%: they're a rounding-error benefit at best; make sure the rest of your pricing has headroom. Below 15% margin: if you can opt out, do; if you can't, your real problem is pricing, not ads — raise prices or cut costs until an offsite sale is at worst neutral. The fix is never "hope ads don't happen".
The cleanest approach for growing shops: build the 15% into your price floor from day one, treating offsite-attributed margin as your minimum acceptable margin. Organic sales then over-deliver, ad sales still clear the bar, and crossing the $10k threshold changes nothing about your economics — which is exactly the point.
Fee percentages and thresholds are Etsy policy and change — verify against Etsy's current Offsite Ads page.