SWT don't seem to have that much faith in the reliability of the 458/5 EMUs, judging by their non-use on diverted Windsor services today (Sat 16/01/16). During a four hour lineside period between Norbiton and Strawberry Hill, only 450 and 455 units were used.
It prevents confusion over which coaches to be in for the short platforms on Down services to Windsor, as the services reverse at Twickenham. If boarding between Waterloo and Twickenham you'd need to be in the
rear 7 or 8 coaches (instead of the usual
front 7 or 8) for the stations with short platforms. As a 455+455 formation fits in every single platform on the route, you would remove a major source of confusion. It used to be worse before the 10 car platform extensions as even 450+450 didn't fit at New Malden, Norbiton and Hampton Wick in at least one direction (I got them on a diverted service in Nov or Dec 2013).
And do stations from Earlsfield to Teddington inclusive have the SDO beacons that tell the 458 how many coaches have stopped in the platform? If they are not fitted then only the front 4 coaches get released.