I symbolic link all my installs to one addons folder. Yes there are addons available for slaves that they won't ever use but that is ok. The system works well for me.

Only updating one folder makes life much easier. Another option you can use , which I used with Jwow ace updater, is copy your wow matrix to different directories x6 and point each one to a different install. You will have to run each one seperately but will still help to streamline it some.

Good luck.