My take on Studio Pro.  In order for DAZ to provide more of the advance options (volumetric lighting, procedural texture,...) they need to access more of the 3Delight engines features.  In order to access those features they need to pay for a better license agreement.  In order not to lose money they need to pass that cost onto the customer.  Again just my personal best guess for a Pro version.  Also there has been some talk of the Pro version possibly being 64 bit.