Tex, nice flashback there, and I understand your suggestions, but prints are not cheap once you get to 8x10" or larger. All big prints aren't posters - most posters are printed large on cheap paper using 4-color printing - large prints are printed on heavy paper using more sophisticated printers.
Also, sure OS-provided calibration tools are better than nothing, but are pretty worthless in terms of getting accurate prints. For $80 or $90, you can assure yourself of accurate prints, vs. spending that much on wasted prints that you're not happy with because they are too blue, yellow, red, dark, light, etc.
Also, I don't see it as diagnosing a problem he doesn't have because an uncalibrated monitor will lead to problems, and the bigger you print, the worse it gets. While most people don't need a full-blown ICC workflow, I think monitor calibration and a printer who uses ICC profiles is the minimum you need for good quality prints. For many years, that's all I could afford, and I never had to re-do a print.
Ok, so in your initial post you say 'Are colorimeters really necessary?", and now you are lecturing me about how necessary they are. I have a Spyder 4 pro, and use from time to time. I think the type and grade of monitor are just if not more important. There you win, you have convinced me! My largest print is 25 feet wide and hangs over the tourism bureau at the local International airport. I did not make the print, they just bought the image. I hope they did not print any 4x6 or 8x10 test first, those can be expensive.
If you want seriously want to argue with someone about "the definition of a poster", you have the wrong guy. I am more the photographer type.