If, for reasons which remain mysterious to me, there is a comfortable and stable gap between Mt. Gox and others, and if one has a relationship with Mt. Gox which allows the kind of turn-around needed to succeed at arbitrage, then it makes the most sense to try to just keep the happy state of affairs chugging along.  A 'stampede' at Mt. Gox would probably spell it's flame-out and thus the end to the milking the arbitrage.