I got over 2000 hours of play time in TF2, trust me, I do know spies. It seems you are talking about Cloak and Dagger. Not many spies I do know use this watch at all, because it drains like crazy compared to the stock watch, and forces you to stand still to reload your charge, which in this game is kinda weird.
Most spies do either use the Dead Ringer, even now with its down sides, or the stock watch, because they are way more flexible and superior compared to the Cloak and Dagger. Yes, the stock watch can be refilled by ammo packs, but it is now also the only watch and you need to have ammo in reach, of course. And playing spy is all about creating mind games, not fancy trick or chain stabs.
A good spy though still has a much bigger impact on the game like a Sombra has, because instant kill is instant.