mysql 5.6 gtid replication: is log_slave_updates required? I've read the docs saying log_slave_updates is required.  It makes sense for a general promotion situation.  

But is that really still required for something you're intending to be a read only slave that will never get promoted?
Yes, you must set it when using gtid, otherwise mysql won't start, providing you the following error:

[ERROR] --gtid-mode=ON or UPGRADE_STEP_1 or UPGRADE_STEP_2 requires 
--log-bin and --log-slave-updates


The slave requires its own executed binary logs in order to auto-position itself in the case of a stop (for example, if it crashes) or a master change (the hardware on the master fails without a possibility of a fix and another slave is promoted as the new slave), even if it is not going to be a master itself never. Although I suppose that can be controlled exclusively in-memory when running, I would bet it needs its own binary logs as a failsafe in case of a crash.

You may think that that is a lot of trouble, but it is a small burden in exchange for not having to deal with binlog files and binlog pos addresses anymore. Also, binary logs on the slave was always a good idea. You can still purge them automatically with expire-logs-days/max-binlog-size if the size is the problem. And with group-commit in 5.6, the performance penalty should be minimal. The other thing why I did not used log_slave_updates before (checking that no writes have been done on the slaves) can now be controlled with gtid_executed values.

In a nutshell, set it up, and do not worry on most cases.