960->1020 isn't necessarily that small depending on what the space is used for.
If it's just making a content column wider then it's not really that useful in most cases, but if it's adding another column with useful information for the user, then it is a big deal - it makes the website that much easier to use.
to give you an idea of what kind of value can be added
Sure adding another column could be useful but as you can see by the authors tone he is talking about flow and divisions of columns and how those go well together and in all reality it sure doesn't make a hell of a difference even if you shrink something to add an extra column. After a while you get cluttered and usability/simplicity goes out the window.
If it's just making a content column wider then it's not really that useful in most cases, but if it's adding another column with useful information for the user, then it is a big deal - it makes the website that much easier to use.