Some people buy gear to meet certain needs. When something comes along that meets those needs better they get it. Especially if their current gear is not adequately meeting those needs.
Others who do not have that need don't see the point and make threads like this. They are not entirely wrong, as I am sure many people buy the new gear just to buy it. I have done that once or twice myself (e.g. I really didn't need to go from the Sigma 70-200 OS to Canon 70-200 IS II since I had a primo sample of the Sigma). However, many people do have a real need, one they sometimes can't even articulate, that is a valid reason for getting the latest and greatest.
For instance my most recent purchase was the 5d3 even though I have a 5d2 and 7d. Many see no point to such a purchase. I do see a point though because it meets my personal photography needs much better. Needs that involve a fast-moving 3-year-old, an inadequately lit home, and frequent situations where I don't have time go grab a flash. To me a thousand dollars more is totally worth it to get those photos. Others lack this need and it isn't worth it to them.