Its one of those things - you have to start somewhere, and sequencing the DNA of individuals with unusual characteristics ( such as long lived people ) is an interesting data set to have for future analysis and comparison.
Its amazing that its only 10 years or so since the first nearly complete human genome sequence was released and that the original projects budget was a couple of billion dollars, whereas now it can apparently be done for four or five figures.
There are a number of projects which are in the process of getting thousands of genome sequences as you suggest, eg http://www.wellcome.ac.uk/Funding/Biomedical-science/Funded-... .
Its amazing that its only 10 years or so since the first nearly complete human genome sequence was released and that the original projects budget was a couple of billion dollars, whereas now it can apparently be done for four or five figures.