We are dealing with two different kinds of vibration.
FORCED vibrations are those that are introduced to an object. Runout or unbalance in a tool assembly will create a once per revolution frequency. It is consistent, no matter what speed the tool assembly is rotating, it will always be once per revolution.
SELF-EXCITED vibration is the third and post challenging. It comes from an external force acting on an object. A tooth on a milling cutter strikes the workpiece and the force causes the tool to deflect. It rebounds back, but if the timing of the next tooth is off it either undercuts or over-cuts changing the force and amplitude of each deflection. This modulation of the cutting depths of each tooth gets worse leading to chatter and tool failure.
A perfectly balanced and concentric tool will not survive the first impact of the first tooth in the workpiece. It is not 100% rigid, so it will deflect, instantly becoming out of balance and off center.
However, a balancing a tool is very important for a different reason...REPEATABILITY. A balanced tool with the same stick-out will have robust and repeatable performance. This will result in longer tool life and better surface finishes at ALL speeds.
NOTE: the SLD and CDD readings on the Dashboard are predictions of tool displacement due to the flexibility and damping of the tool assembly. They are not impacted by imbalance.