The article linked earlier in the thread suggests that in terms of overall mortality this is somewhat true, although losing a bit of weight can certainly help with such things as GERD, mobility problems etc.
"Likewise, we don’t fully understand the relationship between weight and overall mortality. Many of us assume it’s a linear relationship, meaning the higher your BMI, the higher your risk of early death. But Katherine Flegal, an epidemiologist with the CDC, has consistently found a J-shaped curve, with the highest death rates among those at either end of the BMI spectrum and the lowest rates in the “overweight” and “mildly obese” categories."