The reason you don't necessarily want the throttle firewalled near stall speed is because it can cause you to lose control. While I'm sure there's all kinds of aeronautical terms for the forces at work, the way I look at it is simple- When you're close to stalling, there comes a point where the torque of the engine gets closer and closer to overpowering the lifting forces of the wings, causing the plane to flip over in a snap roll. Turning a 190 at low speed will show you this really quickly.
The reason I roll back a bit on the throttle when it gets close to that point is to stop the snap roll while not cutting throttle completely. The rough idea is to throttle back juuuuust enough so it doesn't snap roll. I still maintain control of the aircraft and still am gaining speed, once it gets going a little faster I can firewall the throttle again. It's a matter of familiarity with the plane you're flying, but once you see it it opens up a lot of possibilities for refinement of maneuvers.
Wiley.