Given I can get 40+mpg from my petrol Caterham when on the motorways, the extra frugality of the diesel certainly isn't worth it (for me) when compared to the fun of a high-revving, exciting petrol engine.
I suspect it was done more for the novelty value than for the fuel economy.
Can't think of another reason. All the advantages of the engine are utterly pointless in this car, so you're just left with a heavy engine with no revs and no decent noise. Odd.