I'm guessing that with a 4s LiPo and an UBEC, even under load there's always plenty of power for the UBEC to be able to provide 6V to everything. The receivers not really running anything, the current's just passing through connectors / PCB, or not even that where a Y harness is used.
In 1/10th with only 7.2V or a 2s LiPo, there is much less margin for momentary drops by the BEC below 6V, hence the need for the power cap.
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