For me the two important points are these. First - giving some things a shared name, like calling them a 'disease', helps us categorize and understand those things. Second - Having a suggested list of criteria helps us classify if things fit into that category. The list is useful even if I don't agree with it completely. Having a starting point for a discussion is important.
Calling something a disease is powerful. It implies that the something is worthy of the attentions of the medical and scientific communities. It implies that it is fair game for research in both the public and corporate spheres. Cures, palliatives, and various concoctions to help with the symptoms can be sold and used. It implies that employers and insurers treat the thing differently simply because it is a disease.
So... is obesity an epidemic? Should people be expected to stay physically fit? Is being a lazy slob and a couch potato be considered a lifestyle choice? I still don't know the complete answer but I have something to base the conversation on. That's a start.