Correct, most of the traincrew issues (certainly on the Southern reaches of the WCML) have been driver related. Platform lighting would certainly be an issue - especially at places where there is curvature of the track (Wolverton, Kings Langley, Berko, Leighton to name but a few) as would things like Passcoms, CFA's etc etc. Just imagine, a 12 car sat somewhere like Hemel Hempstead, on the fast, with the slow lines closed, with a passcom activated in the bog of the rear 4, while it's packed full of ****heads. Carnage. It's difficult enough for Guards to get through all the bodies to pop it back out without being called all sorts of rude words. Also, 350's are not equipped with Selective door Opening, so that puts the brakes on calling at places like Apsley and Kings Langley on the fast lines - the platforms there only hold 8 carriages. As with Bushey's Down Fast platform, which is only just long enough for a 4 car!!
Just to clarify to a previous poster, ALL LM services currently carry Guards, including the branch lines. The only services that are worked on a DOO basis are those that convey ECS to places like Camden. Even when 313's worked the DC for Silverlink, they had Guards.
The current rolling stock (mainly 350's, as most people know) are not designed for DOO, although there are door controls for both sides of the train on the Driver's desk. 321's also have these controls on the desk.
I've heard stories that some services were worked DOO on a trial basis between Northampton and Milton Keynes, but too many drivers refused to take the trains without Gards.