I think that anything like MB that whittles a person down to an easily identifiable category can be dangerous only if people take them too seriously. They're good prompts for self-reflection and I've yet to find someone that earnestly answered the questions and then thought that the assessment they were given was wildly inaccurate. Also, it's worth noting that they're all quite different. For example, if I were told I was the personality style that was assessed to crafty, I wouldn't buy it for a moment.

In regard to the five categories the Sales Executive Council established, if you read the book Challenger Selling they make it pretty clear that most people have different gradients of each style. But, it's been my experience that their claim that Challenger's are the most successful is spot on.

As someone that has made a living being able to read personality styles in order to quickly create a buying environment, I know that there is validity to these index tests. Are they static? No. But they can provide insight in to yourself and others. And... like you said, they are fun.