Protection against hardware failure. In my mind 200 devices is better than less devices, as 1 failure only affects 0.5% of hash rate. We have these moving round in stock all the time so any units that die will be replaced with working units as fast as possible. Can't give any guarantee as to the timescale that would be, because it would depend how we are for units in the warehouse at anytime. it will however be first possible opportunity
Ever heard of hot spares?? Why not simply run 4 or so extra miners? There's no loss because there earning, and that's enough to cover failures and keep an active 200 machines mining all the time for the pool.
also, this implies that all your 'warehouse' miners are mining(not boxed up) until they are sold??