Booking codeshares makes sense in some cases. Often both AY and US marketed AY operated flights to JFK cost the same. Those of us booking US coded discount economy tickets earn 100% Avios (also tier points)/AY Plus points while those booking AY coded discount tickets earn only 25-50% AY points or 25% Avios.

For non-status customers it probably makes much more sense to book codeshares as they can't book seats for free etc.

Of course with *A it makes much more sense to avoid codeshares because of the point lottery...