The reason the 'Trigger trick' works is because of the same principle as if you hold your joystick or c-stick a little off center when turning on the console (actually both work the same if you just do it while plugging in your controller), it accepts its position as 'unpressed' and with L or R, if you have it pressed, then there is no in between 'unpressed' and 'fully pressed', it's all or nothing.
Powershielding definitely doesn't remove hit stun. Try powershielding ganon's warlock fist (lvl 9 cpu's love it), and buffer a roll or dodge with c-stick. You can't do it until many frames after.
Powershielding projectiles is extra win, as said earlier. Most characters with a decent WD and grab speed can powershield falco's laser, full length wavedash, and grab him.
The only use I have found for powershielding non-projectiles is that traction either doesn't apply or is severely reduced, meaning attacks won't push you away, making punishing more possible when under pressure.
EDIT: Bumped for knowledge