He's assuming that the designer has a fixed amount of time to work on the project, so less time spent faffing with pixel alignment means more time getting the design right in the first place. I suppose there is also the hidden assumption that if the designer spends more time on usability, then the design will be easier for the user to use. Though from the hostility in your message, I'd suspect that you might not believe that.
Finally, he was talking about designing for a small, fixed set of screen sizes vs designing for a range of screen sizes, so your final point about your monitor size does not apply.