My view was always that feedback is vital and that problems and issues are known as long as they are acted upon and lessons learned, as Roger said " how will things ever improve" if you don't know about it? A disgruntled customer might just walk away but won't come back and is highly likely to slag off the company to all and sundry, we've seen some of that on the forum have we not.
It's much easier and cheaper to keep a customer than procure new ones and if one of your customers has a problem, providing you act quickly, communicate effectively and resolve the issue you will likely have that customer for a very long time.
The customer is very definitely not always right and sometimes you need to disagree with them but most people can handle that as long as you're truthful and transparent.
Just to put my Currys comment into perspective, in my email to the area manager I didn't name the branch manager, I didn't make a complaint, I questioned their company policy of not offering discount and suggested that was handing business to their competitor, which it did in my case That was the excuse I was given in store!. Only during his phone call to me did I answer the questions he asked about the store and it's manager. I see nothing wrong in that whatsoever as even though I'd had previous problems I didn't slag off the manager.