What good are carbon offsets?

Well, in the face of overwhelming publicity, Tony Blair spun on a dime about his travel-caused pollution and how he planned to deal with it. Now he says he will purchase carbon offsets, to balance out the carbon he used in his holiday travel, which is a good move for him politically, but I just read an interesting article about whether carbon offsets are really valuable. Interesting stuff.
LINK to article "A lot of hot air?Can you really repair the damage caused by flying by paying a few pounds to a carbon offset company? Tony Blair seems to think so - but Dominic Murphy has his doubts "