Theres no particular obsession with staying in top gear other than I do because I can, which in a petrol you can`t.
Why six gears? Good question, which I can`t answer as the flexibility of a modern diesel (the ability for instance to do both 40 and 90 in third,fourth,fifth and sixth) and hugely better in-gear acceleration times than the equivalent petrol models render at least two of the gears redundant.
it`s like asking why have electric windows when a winder does the job. or do I REALLY need a ten speaker stereo, auto lights, auto wipers and all the other tinsel (electrically adjustable seats!) that you get whether you want it or not.