Sometimes body positioning alone is not enough to controll traction.
If the front starts to lift and shifting the weight forward dosen't help, don't let off the throttle, just slip the clutch a little. This will happen when you have lots of traction, loamy dirt.
If the rear starts to spin, and shifting the weight dosen't help, slip the clutch a little. This will happen in mud and hardpack.
You're never going to be in the perfect position all of the time, or in the right conditions to be able to use all of the power, the clutch lets you maintain throttle position and concentrate on controlling the rear wheel with only 1 controll device instead of 2. On a 125 your clutch is the controll that you use most, Ideally leaving the throttle pinned the whole time to make sure that the engine is spun up. The last thing that you want is to let off the throttle and drop off the pipe.