Should one apologize in response letters to reviewers? If not, why not?
My rule of thumb, generally in life as much as any specific aspect of it, don't apologize for something you haven't done wrong. This is of course highly debatable but an apology is a formal acceptance of a mistake made, not a figure of speech to be polite. So I'd say don't apologize for something if you don't believe you "f*ed something up". 

It does have a cultural component however, as a British friend of mine once put it:


  buying a bag of crisps in England involves 4 "Excuse me"s, 7 "Sorry"s
  and numerous "thank you"s. 




In this scenario specifically, I think it'd be an insincere apology, and might even be taken as mockery to apologize in response of a comment made by a reviewer. I'd rather advocate for a respectful and appreciative response, something like: 


  Re: "[blah blah blah] should be discussed and evaluated." 
  
  We would like to thank the reviewer for pointing out the lacking
  discussion on [blah blah blah], this has been added in the revised
  version of the manuscript, see page X, paragraph Y.


or, if you disagree with the reviewer about that particular comment because it's not missing


  Re: "[blah blah blah] should be discussed and evaluated." 
  
  We would like to thank the reviewer for pointing out the lacking
  discussion on [blah blah blah], however this topic has been covered in page X, paragraph Y. 


OR, if you disagree with the reviewer about that particular comment because it's not necessary


  Re: "[blah blah blah] should be discussed and evaluated." 
  
  We would like to thank the reviewer for pointing out the lacking
  discussion on [blah blah blah], this topic has been omitted because reasons A,B and C.


Of course these examples are grossly simplified and generalized, usually a bit more context regarding the proposed changes would be appropriate to give in the response. Remember the response is aimed primarily at the editor, not the reviewer. If you can convince the editor, you might even bypass a second iteration to the reviewer.