J.B, Our recommendation is certainly to turn this on in your environment, this is documented in the best practice guide available on support.emc.com  docu31003_FAST-VP-for-Symmetrix-VMAX-Theory-and-Best-Practices-for-Planning-and-Performance  As you have correctly stated it is generally a good thin, I guess at the moment the reason it's not default is that there are still previous versions of code out there that don't support it.  Perhaps when 5875 goes EOSL and a few revisions have passed we will see this be the norm.  It is not a replacement for monitoring your Pools but it will certainly give you an added protection, it will also ensure optimal placement of data if performance data already exist for the track groups so should cut down on some back end activity.
This new feature within FAST was introduced with Enginuity level code 5876. I am interested in knowing why this feature is disabled by default? From what I read the feature would benefit any environment from write failure.  Thanks in advance!
Allocation by FAST policy