It's also much easier on the user (and in turn, easier on the designer to make it easier on the user). Knowing the exact surface area each control will take up, the exact distance between elements as they will appear in the real world, gives the designer that much greater degree of precision in crafting a human-centered UI.
Please elaborate on how it's easier on the user. I agree it's easier on the designer (though I disagree that that's a good enough reason NOT to use a fluid layout in this day and age), but I fail to see how a design that refuses to conform to my monitor size is any easier for me.
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.
I think you mean "an insufficient amount of time." Some finite amount of time should be enough to come up with something quite good for any finite interface with reasonable requirements.
(And an infinite amount of time still probably wouldn't be enough to come up with a good interface based on bad requirements.)
Why do you need to know the exact distance? If the aspect ratio is fixed, it seems like exact distance is just a function of resolution. But you can define things exactly where you want them stil..
I would think it would be to make sure to control the actual physical distances. You are more concerned that two buttons are 10mm apart then 10px apart. If they become too close physically, then the higher the chance to miss click or other difficulties.
