Well by "corrupt" I mean to alter it from its original form or intention. Probably not the best choice of words I must admit.

"Decoupling" is a very useful tool. I think Joel's point of view was that unit testing often takes it to an extreme where the work to manage it outweighs the benefits. I personally don't agree with him. I think the benefits are too great to ignore. But I can certainly empathize with where he's coming from.