It depends really. If the weapons are well balanced so perhaps the shotgun excels in one area but is a bad choice in other circumstances, then it should be fair game. If it's good to the point of being the best option by a wide margin in every situation then it's stupid to allow it's use in competition because it actually limits potential strategy to one option: use the shotgun or you'll lose. Kind of like not allowing Akuma in the original Super Street Fighter II Turbo makes it feasible to win with another character because he's actually that broken.
Now I don't know enough about Halo to know if that's the case, but it sounds more like they're trying to limit the potential options so that there's only one option available to everyone and the competition comes from reflexes, and luck. Limiting a game full of options to just one is stupid because you're either removing viable options, thus reducing the use of strategy and tactics, or the game is so poorly balanced that you have to do it to make it marginally viable in a competitive setting. If it were the latter case then you might as well just compete in a better game.






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