Well, I had the same question regarding this frame as I am planning to psh the trigger.
I am ridding a Niner with the CVA suspension. Only on a sprint I would lock the rear, other wise, Trail or Open mode all the way long. But, the suspension is very efficient despite what peaolple think of Niner.
On a Scott Spark, I would want a lock, what ever you can argue.
I guess, the feeling of the bike is also important. I am racing my Niner Open mode or Trail mode according to the race. Stiffness is not its main ability, but it is bloody efficient on rude terrain. The Spark is very fast on fast rolling races but bloody exhausting on technical trail despite the slacker head angle.
This FM936 seems very rigid around the BB, so I would go for a locking system to put down the Watts on fast terrain.