When you’re tightening a tweaker, you’re closing down the leech. There’s potentially a use for tweakers, but sail designers design the sail so that the boom ends up working like a tweaker. This is very important – the lower you go, and the more you ease the main, the more it acts like a tweaker, putting downward pressure on the spinnaker sheet and closing the upper leech instead of letting it open. So, the mainsail helps the spinnaker fly more efficiently.
The only reason you’d use one as the boat gets bigger is to keep the sheet away from the boom in light air, where the boom can come inboard due to the pressure from the sheet. However a tweaker brings the bottom of the sail closer to the boat – you want the spinnaker away from the boat. If you can get away without using them you’re better