I believe EU261 applies to any airline flying to/from an EU destination.
In fact it applies to any european airline and for flights (by any airline) originating in the EU.
So for an AC flight departing MAN it does apply.
First of all passengers want to know their options, they want to know when their fliegt goes, of if not, who else can carry them to their destination or whether it is time to call it day and go home not travelling at all. Travelling to a wedding, funeral, meeting or an interview 2 days late does not make any sense. In that case you also want to inform the people on the other end of the pond that you will
not show up, at least not on time. You need to cancel hotel reservations (or you have to pay for the night you do not use, and lose the reservation for the others...).
All of this could be managed by a well organized cutomer relations agent, or you leave your customers to their own devices, and hope they chose your company again.
Anything can happen, but at least you should be able to handle it properly.