Being able to selectively cap the movement speed of one or more selected unit(s) provides an opportunity for feints within radar coverage of your opponent but outside their direct vision. 10 Radar dots moving at the speed of a Bull (but are actually speed-capped ticks) showing up along your flank may be able to draw forces away from the main group and allow for a defeat in detail.
Could also be used to cap the speed of Radar/Jammer bots so their movement can be queued with the rest of their group (Sharpshooters/Sheldons/etc) without additional micro during movement (the setup itself is micro so it would be a situational tradeoff if you plan to move that grouping often)
Ideally it'd use a slider and be per-unit, not per-unit-class. Maybe you could change its default value for produced units in Advanced Settings.