The bottom line is we all use tricks to motivate us to work harder during workouts, and racing is a mental as well as physical game. I do not disagree with the arguments about physics but the facts are these in my little world:
On the flats and with a light bike and no power meter, it is very possible to convince oneself that the workout is hard, when, in fact, it is not.
Hill training is particularly useful because hills force us to work hard- the gear ratios are not infinite, we cannot balance well at 0.5 mph, etc. So to make some hills, we have to work hard or fall down.
If we combine some hills with additional weight, we'll be forced, in effect, to use a larger gear than normal to make it up the hill (because we cannot find an easy gear). The human will to make it to the top of this hill, with the gears I've got, is easier to tap into than, "I will maintain xxx watts for yyy period of time".
Harder work makes us stronger. We do whatever it takes. A backpack full of rocks going uphill works. Working out with a training partner works. It's not all about the math, much of the struggle is to find ways of making ourselves do the volume and intensity of work required to get stronger.