You can actually have NO plugs or trick fueling devices at all in a 2 line system on a cowled airplane.

Beginners, there are a LOT of plumbing strategies. This discussion actually is confusing a couple of different ideas that have nothing to do with each other.

Why use a 3-line system? Because Uniflow needs 3 lines and Uniflow is worth having to solve problems that happen with some airplanes. Or because you've got a cowled setup and want a carb pickup line with no interruptions. And you don't want to buy a device that overcomplicates a system that doesn't need it. Or a number of reasons.

I've got a couple of models that have benefitted from having Uniflow. I fly the hell out of them and the engines run significantly better with Uniflow. How do I know the engines run better with Uniflow? They started out without it. Do you need it? Probably not. Not yet. But if you ever do, it's there to try. It might help, it might not.