RocksDB is technically only for account history. (It won’t have an impact on witness nodes.)
Separating it out from the current “master” branch could introduce new risk into the code they have already been testing for weeks. It may be more trouble than it’s worth.
I also don’t know what real benefit it will provide since it sounds like all the node operators who are running account history will probably want to pick it up as soon as it is available.
Also FYI, some witnesses (myself included) have been running the RocksDB plug-in on some test nodes for a while now too.
Oh, I just meant purely a naming/branding change for the final release. Anyway, not an issue either way.