He won't lose any money in slippage if he puts up a limit order (it might sound imbecile to some, but most people do place market orders, including the ones that think it's imbecile).
People rarely change defaults... how about as a first step making the default a pre-filled limit order routed only to Bitfinex? It would be nice to set the "only to Bitfinex" default to something else in the account settings of course. Not claiming a BTC for that one, but if you think people just quickly clicking buttons are a problem, do something about it!
Customers will then execute market orders and due to the fact that it's pre-set only on BFX they will blame on us the fact that they lost money due to slippage (it's always our fault when they lose).
Raphael wants the customer to always pick by default the best possible option ( he even embedded commissions in the price to make things easier) and I must say I share his view.
Thank you anyway, nice try

Giancarlo
Bitfinex Team