Smoking takes 10 years off your life - but is this a sufficient reason to give up smoking? Why is a long life a better life?
The United Nations Human Development Index uses life expectancy as a measure of life quality because:
a long life is valuable in itself and... various indirect benefits (such as adequate nutrition and good health) are closely associated with higher life expectancy
That longevity is a proxy for the things that make life enjoyable is a reasonable argument, especially in the context that the UNDP is making it. The elimination of polio, for example, not only increased life expectancy, but also improved life quality, by preventing people from experiencing years of disability. But smoking shortens life by taking low quality years off at the end - is that such a bad thing?
And the statement that "a long life is valuable in itself" takes us back to the original question: why?