Should threaded pipe be torqued to spec?
Torquing a bolted joint serves a very specific purpose - to set the clamping force exerted by the bolt on the joint.  By applying some specific torque to a bolt with a certain thread pitch, the effect is to stretch the bolt by a specific amount.  The bolt then acts like a spring, clamping the joint together.  Overtightening the bolt applies too much clamping force and can actually deform the joint.  Undertightening means the joint can't take as much force before it "floats" and fails.  

Threaded piping is completely different.  In this case, the threads aren't used to apply force to a joint, the threads are the joint.  Their primary purposes are to keep the pipes from separating under pressure, and to seal the cavity inside from the outside.  For strength, tightness is of little importance.  For sealing, tightness matters, but no matter how tight, the pipes will never seal without a soft interface material like dope or tape.  Therefore, thread pipes need to be tightened until they seal under pressure, and no more.