It will reduce the training requiment in the GA sector as the MPL does not require the pilot to fly single crew except right at the start its then straight to Sim School (Ryanair?) for your multi-crew pilot training.... (There is no requirement to fly light single crew twins and the Multi-Crew Instrument Rating is done in the multi-crew environment
As Dylan says the driver is the airline industry who expect Pilots with an initial AIRLINE pilots licence to have flown an airliner. (naive I know)
See
http://cat.texterity.com/cat/2005-4/?pg=11
Look to 2006 for some movement on this see...(page 21)
http://www.halldalemedia.co.uk/eats2004/woods.ppt