What Motion Compensation Removes from Accumulated Radar and When It Still Helps

Accumulating radar sweeps requires choosing where to place past returns before spatial rendering. Doppler-based motion compensation can concentrate returns around moving objects, but it also alters spatial trails that may support velocity estimation. We examine this trade-off through geometric measurements, linear probes of frozen encoders, and controlled detector comparisons on nuScenes and TruckScenes. On nuScenes, compensation reduces the correlation between normalised trail extent and object speed from 0.764 to 0.297 over ten sweeps, while increasing in-box return counts for fast-moving objects by 54%. A displacement-matched permutation reproduces neither effect. Object speed is more linearly accessible from raw-placement features, with an $R^2$ advantage of 0.019 to 0.032 across eight frozen encoders. In detector comparisons differing only in placement, raw placement yields lower velocity error in eight of nine paired comparisons across three systems and three training seeds, whereas compensation provides less consistent detection benefits. On TruckScenes, compensation reduces aggregate in-box concentration across moving objects at every evaluated accumulation window. Near-radial objects are the exception and the locus of both effects: there concentration instead improves and the trail-speed correlation is removed, an association with radial motion fraction that we do not explain. We further decompose the end-to-end gain of a detector retaining both placements. Along the evaluated sequence of controlled comparisons, correcting a Doppler coordinate-frame inconsistency in the reference pipeline evaluated here accounts for 69% and 81% of the mAP gain at two seeds; a displacement-matched, correspondence-breaking control recovers part of the remaining gain. These findings show that spatial concentration and access to motion cues can favour different placements, and that attributing downstream gains to placement requires controlling for changes in input features and architecture.