Why is druid not supporting shard-segment merge by default?

Hi, all

Recently I has been looking through the source code of druid on 0.10.0, and of course reading many discussion among the google group. To learn that druid currently does not support shard-segment merge, but all the discussion never point out the direction why is druid not supporting this feature. is there any one could be kind enough to state the reason why?

This feature is work in progress. See discussion at - https://github.com/druid-io/druid/pull/5102