Because you can't crop, of course. Exactly what you said. People who use the Image Size method have to use a two-stage processing - cropping first, then Image Size. Ain't nobody got time fo' 'dat.
That person is an idiot. Pay no attention to them. When using that method, you don't have the handy handles at the corners and sides of the marquee, for tweaking your crop. Plus you don't, as you mentioned, have the control over size and resolution.