A more secure way would be for the sword winner to inform you of his or her character name, and have the winner offer something like rocks for the sword. This way there is no chance of someone Ninja-ing the sword away from the winner.
Oh, and um...
Teamwork is important because we all love the experience and xp gained, and thus the better your teamwork then the better the chance of winning a larger reward! Teamwork is also important because with a group of friends or even randomers, it can make the match much more enjoyable. The last reason why teamwork is important (the last reason that I shall post) is that people ridicule you if you play poorly and that can be bad for some people's egos, especially if they have the same banner and are stuck with you, as well as that greater teamwork means you shall play longer and thus have more fun!