Another reason to put away electronic devices is so that you are tuned in during critical phases of flight (takeoff, climb out, approach, and landing) should any emergency arise.

While you may be able to poke holes in the rules by citing specific examples where the rule perhaps doesn't make sense (i.e. airplane mode vs. completely off, standby vs. shut down, Kindle only consumes power when you change the page), the rule also has to be simple enough to be enforceable. The FAA can't provide airlines with a rule carrying 1000 exceptions and caveats, so they just create one rule: no electronic devices.