why do we still use magnitudes?

As my loyal reader knows, I am writing a piece on magnitudes, distance moduli, color indices, color excesses, bolometric corrections, and so on. Today I sent a copy of the draft to my excellent colleague Mike Blanton (NYU) for comments. He came back with lots! One of the main points he made is an elucidation of why (in his opinion) magnitudes make sense to be using even in this day and age: The physical quantities of interest about a (say) star are its bolometric or total luminosity, and its detailed (high-resolution) spectrum. The thing we can observe, photometrically, is neither of these. So we integrate photons in a bandpass. The magnitude system is a way of summarizing the choices we make when we do that, and reminds the theorist or interpreter that our measurements are complex integrals of the things of interest. That's a good argument. Prior to this feedback, my answer to the “why?” question was about precise relative measurement, which isn't so relevant in many contemporary contexts.