Well, Srijit there's no hard and fast rule when it comes to finding the sweet spot in the rev range in which we should drive our cars. Turbodiesels, infact all turbo engines are at the peak of their efficiency when the turbo is at the required speeds and producing max or close to max boost. That ensures efficient and clean combustion. Soot harms diesel turbos as much as high heat. When lugging, the engine is not at it's peak efficiency and as ItaliaLinea has pointed out, the fuel consumption also increases. A corresponding increase is in the soot levels which is hazardous to the Catlytic convertor as well. Clogged up catcons reduce performance. A nice hi rpm thrashing at times is needed to burn all that accumulated soot. So, as a cheap insurance and good practice , it's good to drive our engines where they feel most comfortable and happy. Cheers.