If you break the neck of merc that merc gets re-equip of their gadgets. Yes 4 new nades or 3 new mines :F but if you KO they won't get re-equip and if merc is KO:d near you you see him waking up and thus smoke or run.

Good question, typically the neckbreak is too attractive as the merc loses about 10 seconds if you hold him, maybe 5 seconds dying and 10 seconds respawing. That's not even mentioning making his way from wherever he spawns. The knockout works best on rush maps where the spies can win by reaching the objectives first like Bank or Warehouse maybe.

Again when u KO next to terminal the merc can't surprise you as you can see him waking up and won't get re-equip of gadgets

It's a good way to show off like in a "I don't need to kill ya" kind of way.