Which bricks warrant an extra to be included in a set?
The ones that come with extras are almost always the smallest pieces in the kit that would be easy to lose and are negligible in weight.

From gizmodo article of LEGO FAQs:


  Why did I always have pieces left over when I built my castle sets? Do they just do that to jack with kids?
  
  For two reasons: first, because some pieces are so small that they weigh too little to be measured by their scale, during packaging (you will see this when I publish the factory tour. J.) Second, because it's better to have too many of those pieces than have one of them missing. Since we statistically know what pieces may get lost, we include some extras when appropriate.