Interesting question.. .. .Come to think of it, its probably like flying any old analog plane (727, 737-200, DC 8, DC-10, 747-200 etc.).. .Actually you can even fly MNPS over the pond with just 2 IRS (among other things).. .. .What do the limititions in the manuals say about this?. .What does your MEL say?. .Youre RNP values will go up, and youll have to hard-tune VORs etc., so you cannot just let it fly on the green line (you never can anyway, without periodically checking).... .. .Do you see any potential hazards?. .Have you had any incidents?. .Have you talked to your safety pilot? (Do you have one?). .. .What must be the risk/benefit/cost- calculation of the people flying desks who let this happen??!!