If a client has MRP, yes Projected Demands may be included in the PA analysis. That is the only advantage of having MRP as it relates to PA. If a client does not have MRP, then the projected demands may be represented by prospective sales orders.
PA is a replacement for MRP, because it considers inventory levels, reorder points, open purchase orders and open sales orders, as well as the bill of materials levels. It is much more accessible than MRP in terms of ease of use, is faster when working with large volumes, and does not impact any other operations.
Going forward, since MRP is not planned to be converted to business framework, PA will serve as the MRP function with compatibility to current versions. Although not in SQL yet, this is planned even though it may be awhile.