According to bungie, tests showed 3 is the ideal team size. The makers of left4dead and the upcoming evolve pretty much said the same thing as the reason for only allowing 4 on a team
Also don't forget that in destiny, the team size reflects the ideal grouping of the 3 different classes, so a group utilises each classes strengths - much the same reason for the group sizes in left4dead and evolve, where each member has a specific skill
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There may be only 3 classes, but remember there are 2 different sub classes for those 3 classes.
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Those very tests are full of shit. They don't at all account for countless things just the class strengths. Team must be 6 tops... 6 in crucible... no reason 6 can't run together
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Which is fine for strikes. But for patrols and rumble ffa that reasoning makes no sense. When you can patrol and run into another group of 3 and all participate in the same public event or mission beacon.