I don't know how the up-front cost of procuring stock that's fitted with dual-voltage equipment compares with the difficulties of having to re-allocate stock multiple times, but it seems fairly logical to me that any fleet orders that are being placed for franchises where multiple voltages can be used, or where there is any likelihood that the units would be transferred to a franchise using the opposite type, for them all to come dual voltage. It'd make the flexibility of switching stock around so much easier, even if the retrofitting process is fairly simple, you can only have so many units out at a time (usually).
To be honest, I'm a little surprised they bothered with the DC-only version of the 378, but presumably they were confident that they had enough dual-voltage units to cover such eventualities - I note that the three extra units at the tail end of the order were dual voltage despite (if I recall correctly?) primarily being acquired to help cover the ELL Clapham Junction route.
Jcollins: What's the source on the Windsor, Reading-Basingstoke, Reading-Newbury, Maidenhead-Marlow and Twyford-Henley routes being electrified? All I can find after a brief search is the main routes that IEP will serve, and the line to Oxford. Electrifying the Windsor & Eton Central branch seems a little bit wasteful perhaps, unless they'll need every last 165 to pinch after they start cascading them?