The move also has a 1 out of 8 (12.5%) chance to be a spontaneous misfire, that acts like if it was fully charged, but more powerful. It's not always beneficial, however. One possibility is Luigi charging the Green Missile and aiming at an opponent. He fires, but the misfire applies and the opponent dodges. This can very often prove hard to get back onto the stage for recovery. It also leaves the Luigi exposed to edgeguarding.
Pikachu and Pichu have similar attacks to the Green Missile, called Skull Bash. The biggest difference is that they can not get the misfire. Also, Pichu's Skull Bash has a much longer charge time, and while Luigi's can go up to 25%, Pichu's highest point is 40%.
For some reason, computer-controlled Luigis will always use Green Missile to recover, instead of Super Jump Punch.