Say you are skiing and lose your balance, accidentally shifting your weight far forward. Say also that you are beginning to fall to the left. You body's geometry and your motion causes your skis to tip to the left. On an expert ski that is stiff in longitudinal flex a goodly amount of your weight will get transferred to the very front of the ski, and due to the shape of the ski the ski's edge (or the entire tip if you are in soft snow) will dial up a hard left turn. Your skis will turn left faster than you are turning left. You will trip over your skis.
Now say your skis have very little torsional rigidity and you are on a groomer or hard snow. Your edge will try to make you turn hard left, but because it has very little torsional rigidity, instead of digging and and turning hard, it will give a bit, such that the tip is not as tipped as the under-boot section. The edge won't grab as hard, and you will turn less. You will turn less whether you are locked in or not.
The ski is trying to turn you where you don't want to go, and you haven't the skill to make the ski try to do something else. If it is a beginner ski you can just force it to move where you want it to, and rather than hold the dialed up line, you can push it around. If it's an expert ski, that is a lot harder.
A ski that is too stiff in will not bend easily. Tipping and bending skis is how you turn. If it's hard to bend the ski, that could set back your learning process. If it's hard to tip the ski, that could also set back your learning process. On the other hand, if the ski has no rigidity, you won't be able to make as tight a turn at any given speed, and the ski will be unstable at high speeds.
You may say that you will put up with the lack of forgiveness because you are committed to getting better. However with the expert, hard to bend ski, you need to be going fast to make it bend a lot (remember you have to bend the skis to turn), more bending equals more turning, and the less you weigh, the faster you need to go to bend the ski. It's harder to learn when things happen quickly.
Balance between torsion and longitudinal flex is also important, but that's enough for now.