Is the no negative language rule a double negative?

I am blaming it on The Secret.  In the last month, we have had two different clients ask us to go back and revise something we’d created a while ago and they’d already been using because it contained "negative language." 

One even referenced the book and the Law of Attraction.   

Now don’t get me wrong…I’m all for the law of attraction. I agree with it.  But I also think its more of a concept than an absolute. 

Then, the topic came up yesterday in yet another client meeting on a current project.  So this has been on my mind quite a bit lately.

As a writer, I strongly believe that sometimes it is more powerful to speak in the negative.  Maybe I’m wrong.  Let me share with you my thoughts and then I’d like to listen to yours.

Words like never, don’t or won’t can be potent triggers. They can connote a depth of conviction, in my opinion.  I like to use them as a contrast.  To give the copy a twist or really pound home a point without pulling out the hammer.

"We’ll never charge you for the same repair twice" is a more powerful statement than the same concept written in the positive.

We don’t do bankers hours feels stronger than we have evening hours or we’re open until 7. 

Our viscosity standards will never be compromised is pretty tough to state in a positive way with as much conviction.

Or am I wrong?  I’d love to hear your thoughts about using negative language in copy and taglines.