I think this is definitely and primarily a marketing exercise by Audi to sell more diesel cars outside of Germany.
What always struck me as strange was that Audi never used their victories at Le Mans in any of their advertising campaigns at least not here in the home market. A high profile entry into the world's biggest and best motorsport event with a diesel powered car is going to look very good on Eurosport etc. and should clock up lots more sales for their TDI range abroad.
I agree with many of the comments here, that Audi were always really clinical in their approach and lacked the soul and passion of teams like Pescarolo who were in it for the 15 minutes of glory on Sunday after the race. The R10 is certainly not going to win any new fans at Arnage corner on Saturday night - it'll be there and gone around the corner before anyone knows it's there! Nor can I imagine the tribunes on the start finish straight full of fans wearing "powered by diesel" Audi gear.
The news that Peugeot is also planning a diesel entry is also a worry - in the long term LM could become a diesel dominated event which would reduce its appeal for the true fans and eventually it would become a curiosity event instead of a real race.
Oh dear, maybe I'm too old for all this......but I can't imagine going to LM and not being woken on Sunday morning with the crackling and banging of lots of backfiring engines rattling through my poor hungover head...