There has been some previous discussion of the association bonus, c.f.
Can we please get rid of the association bonus for other SE users coming to MO?
I've just had to waste some time writing moderation messages to someone with no MathOverflow questions or answers; I wouldn't have had to if we didn't give the association bonus. I have seen no evidence that the association bonus has benefited MathOverflow.
Update (S.K., 04-Jun-2016): Meanwhile this query shows 6 users who have cast more than 500 votes, but have never posted anything on MO. The most active of these voters has cast so far 1367 votes.