I am not an aerodynamicist either, but from observation it would appear than fans (small cross section, high air velocity) tend to have shrouds, while props and rotors (large cross section, lower air velocity) do not. So I would guess the importance of shrouds to the quad-copter depends on the extent to which the blades are fans rather than rotors...? In general engineering terms, I think open bladed impellers tend to be less efficient than shrouded impellers due to the way that shrouds and casings can channel and direct more of the air in the right direction rather than the wrong direction.