If there are gaps between the end of the blades and the cooling shroud you will loose some efficiency as the air spills off the ends of the blades and as the higher pressure air in the shroud scroll pushes back through the gap, short circuiting back into the fan. The pulley probably acts as both an end plate or fence for the fan blade tips to stop spillover and as a seal between the fan and shroud. I think the pulley probably helps flow in this case. Now, whether or not you need that flow is another matter, since you already plan to alter the flow by removing blades.
At least that's my guess!
BTW: I think the fan in your photo is from an early T4 before VW started adding drive accessories to the fan. That nifty little aero cone shape might add a CFM or two!
Mike