You lose any flexibility in your trip after you take your first flight in your itinerary. If booked as 2 one ways, you retain your flexibility to make changes to your return..
Also, one stopover is allowed on each itinerary, one on a return booking, and one on each of your 2 one way bookings.
Return booking your charged for the highesst level of service you received, even if one of your long hauls is in Y. One ways, your charged for your longest sector.
Disadvantages:
Two £25 canx fees if you want to cancel your whole trip.
Certain countries taxes are very expensive in comparison to return bookings - I'm not sure which these are, possibly India?