Getting to 40 is a side effect of doing this. Most people do it for the gear/rewards. That said, getting to 40 quickly isn't ruining anything, at least in my opinion. It's going to happen regardless and let's be honest, it's not like the last 2 dlcs had some major long campaign intertwined in them. They've actually gotten steadily shorter, with HoW being almost laughable. I'll admit, TDB had at least one challenging mission if you didn't have a gally (I didn't at the time.) I don't expect TTK to be anything special in this regard and even if it is it's a matter of personal preference. A lot of people are going to try to gear up as quickly as possible because they want to be ready for the raid when it drops. For many people, endgame is the real game and all that really matters.