No, the small amount of coordination that the primary has to do is pretty cheap compared with the job of actually indexing the documents (tokenisation, analysis, etc) and getting the data safely onto disk, which has to happen on each shard copy whether it be primary or replica.
It's a good point. This setting determines which shards may be moved in order to rebalance the cluster, but this notion of "balance" doesn't distinguish primaries from replicas. I'm not really sure why you'd want to only permit the movement of primaries for rebalancing purposes. This feature dates back many years. I will try and find out.