Limited loss of Y seats
There is a fairly significant reduction in Y seats on MIDO etc. The config for these a/c is C20M108, as opposed to M156 for the rest of the 320s. This gives a loss of 28 seats altogether (and a potential loss of 48 Y seats depending on config). The business seats would have to be sold at a substantially higher price to make up the costs I would think...
Although loads aren't exactly impressive at the moment, I'm sure it isn't the most profitable config except on the MH routes where the demand is there to pay pretty high fares for the swanky Business seats.
It can also make aircraft substitution a bit of a nightmare - having a standardised fleet is helpful.
Having said that it was always nice to get DO/T/Y on the JER-LHR hop. You could fill most of the flight by playing with the recline function!