I spend some weekends in LA on work and sometimes go out to a spot where like fifty guys show up to play every Saturdays and when I'm home in NYC there's another spot I go. Both places fill up with guys whose lives revolve around their weekend pickup games. 1s and 2s are the norm in both of those spots. Of course that doesn't mean it's the norm everywhere or that it should be but I can see the logic to it. Pickup basketball, more often than not, is either played on half court or a smaller than standard size court - which means it's a bit more difficult to space things out. By overweighting the three you make it important for the floor to remain spaced rather than collapsing into the paint.