Well, do you think that too much wizarding pure blood could be a bad thing? There are aonly a few pure-blooded wizarding families left so there must be lots of in-breeding -- cousins marrying cousins, etc. I would think that would be a bad thing. Wouldn't it weaken their magic over time? That's why in-breeding isn't allowed. It can cause health problems in the future and such especially in-breeding over a long period of time It is really rather telling that the most powerful wizards we've seen from the younger students are not pure-blooded. Harry's mom was a muggle-born and Hermione is muggle-born. I'd say that they are probably two of the most powerful students. And Voldemort isn't pure-blooded either. So perhaps, certain families insistence on remaining pure-blooded will actually be their downfall. I wonder if there are a larger number of squibs born to pure-blood families than families that are mixed.