I hear you on that point and have thought the same thing many a time. What irks me even more is that then the brush is dirty, and you have to pull out the thinner to clean it, so I sometimes skimp/cheat on this point and just use the stirrer (usually a flat toothpick) to apply the paint.
I've also sometimes used unwrapped paperclips to stir paint... they're a little bit harder to hold, but the metal doesn't absorb paint like wood so that less paint is going to the trash, but plastic shouldn't absorb that much paint either.
The ultimate solution, albeit a very expensive one, would be to purchase a magnetic stirrer and a bunch of Teflon-coated stir bars from a scientific supply house like Fisher Science; just drop a stir bar in each jar of paint, give 'er a crank before using, and then leave the bar in there until the jar is empty, at which point the bar could be cleaned and reused.... I've seen bars the size of a grain of rice, but paint would probably require larger bars. However, seeing that most bars cost more than entire jars of paint, it'd take you decades (or more likely, centuries) of prolific model painting for this solution to break even... you're better off to just keep doing what you're doing.