I'm playing the German language version of the game, so C'mon = Zu mir! (To me) and Booyah = Cool!. In other words, the translation interprets the Come On from the Japanese version (I'll just assume that they translated from Japanese and not from the US version, considering the rest of the text) as a shout for help, basically. Although I never use it in a dire situation, way too busy trying to survive x'D I usually use it when I broke through somewhere towards the enemy base while my team is in the back, asking for some backup to push. Sometimes it works, sometimes it doesn't. As for Booyah/Cool, I use it to encourage or thank my team mates, when they save my *** or revenge me or something like that.
In fact, I do believe the "C'mon" command is supposed to be a cry for help to begin with, I have seen one or two other translations interpreting it similarly. Though, at least to my understanding, "C'mon" is rather arbitrary as a command in both Japanese and English...