Cathal, I completely agree.  There are many blogs and sample applications to prove this.  However, in the instances where StringBuilder is not efficient, String.Concat is FAR more efficient over the standard concatenation method ("&" or "+").  Even if you do not notice the difference from a single concatenation, the difference over an entire application can be tremendous. 

It was pointed out to me in Gemini that the compiler will simply switch the concatenation characters to the String.Concat method.  However, doing it beforehand saves the compiler from having to do that.  In essence, you save the processor, the compiler, and the end-user time for every single instance of concatenation that is not using the standard concatenation characters.