If the charity didn't think it was despicable, why should I? There is such a thing as abuse of power, but it would imply some sort of suggestion that if they didn't pay Cherie her husband would pass a law banning them or something. They invite her along because they think rich people will come along to listen and donate money. If they didn't think this, they wouldn't have to book her. It's all voluntary, all around.
If you want to see nepotism in action, look at 'Sir' Mark Thatcher. I don't see any evidence of anything like that with the Blairs.