^^^^ Cannot speak to the Obsethed per se, but very much agree with the idea that not all moderate flex (I notice that no one ever says "soft" anymore, too many negative connotations) skis have to be bought long.
First, keep in mind that a longer ski won't necessarily be stiffer, it'll just take more energy to deliver the same pressure to the tip. So that can feel like greater stability if you're running at speed. But that length may be more of a pain, not less, to control or turn because the tip will still get jolted around at speed, and you'll be less able to do something about it with any mechanical efficiency.
Second, IMO this is mostly about style: Do you typically make more turns with quick reactions to the ski's feedback, or do you ski flatter and like longer arcs, and prefer to let the ski do the work? And how much powder will you be in, which will determine a rocker's functional running length?