Does Marco need to flap his wings to fly?
I'm pretty confident that the reason here is indeed limited animation and not the actual way Marco flies. In exactly the same way, you don't see Monet flap much her wings in episodes 612-613 for example.

It might result from or just be confused with laziness, as you mention. Don't forget, though, that anime isn't just a story, it is also the entire team behind it, their short deadlines (often one episode a week!) and limited budget, artistic considerations...


  Originally limited animation was not only a budget-saving choice, but
  a stylistic one as well.


...cultural aspects and a few more relevant parameters, all of which influence the final product.