Having it out will soften the rear up, giving you more lift and flex, which in many cases does free the kart up. Putting it in stiffens the rear slightly, correct. You can also run it with just 2 bolts in as well, to give a very very small change in stiffness from the 4 bolts.
In lower horsepower classes, it isn’t uncommon for people to take it out completely and never use it. Personally, I always keep it in because it doesn’t hurt to have an extra tuning tool. Plus, I need the weight.