We turned it on and it did it's job, unfortunately it did too much.
MPthreading ask for multi-processors on MVS and it got all of them.
Subsequently that MVS image didn't do anything but ADSM.  I submitted a
requirement to IBM to allow the MPthreading to have a throttle of maybe 1, 2,
etc. instead of just yes or no.  Depending on your environment it can be
helpful or cause problems.  

I did this in conjunction with performance testing and what I found was it
didn't help for single transaction throughput, more for several processes at a
time.  We can talk environments and compare our with yours if you think it
might help.  OS/390 runs in a sysplex at Principal so your configuration
should be considered when using this option.

Scott