I think the idea behind PBS is to get n00bs away from over-aggressively attacking and out of a a wholly submission oriented mindset. The sub is the flashy, sexy finishing technique. Its the grapplers KO or Homerun. Guard passes, sweeps, and positional control simply dont have the allure of the sub. How often have you seen newb grappler in class going for the sub like a spazz/madman? And how often do they actually get it? Usually they just end up gassing and getting subbed themselves. A more seasoned grappler knows that a submission is usually the end result of a sequence of techniques that has maximized their advantages.