Why is it important for AB blood types to donate blood?

It's important for anyone who can donate blood to donate, but the fact is that AB is not a donor for any blood type but its own. AB can only donate to AB. AB is, however, what is referred to as a universal recipient, meaning that someone with type AB blood can receive blood from AB, A, B, and O blood types. This being the case, it wouldn't seem that a shortage of AB blood would be a problem unless there is a generalized blood shortage of all types (which there usually is, so donate if you can spare a pint!)