Collective Friction.
At very low friction, there is a real danger of either a pilot-induced vertical oscillation, wherein the pilot alone with the control system enter an oscillation that can be neutrally stable, or destructively divergent. In other cases, the pilot can interact with an airframe harmonic, and again, this oscillation can be quickly ( and scarily ) divergent in nature. The frequency of these oscillations are usually in the 3-6 hz range. The designers typically attack them either by installing a friction adjustment ( hopefully with a minimum setting in the 3-4 lb range ) or installing a friction block of some sort directly in the control system*. The design challenge is to have friction that is consistent and non-ratchety.
* And sometimes, there is enough system friction related to the particular design, to suffice.
Thanks,
John Dixson