In my company we have recently changed from bypass doors open in icing conditions to bypass doors open all the time. The reason for this was that apart from the obvious ¨you can´t forget it then¨, it is more cost affective to leave them open all the time. It appears that the cost of the loss of 2% power is less then the cost of installing new doors after XXX cycles. Long live the beancounters!
Having said that though we do close them at the end of the day to avoid FOD on an overnight stop.
Regards.