Why would people paint rusty metal without sanding and priming first? Plopping a subwoofer down in a convenient spot and pressing the magic room correction button is a lazy band aid that most likely will not yield the best results. Say in the first spot you have a peak in the response right next to a null. The room correction will try to boost the null and cut the peak. Boosting to fill a null (or boosting of any kind) puts strain on the amplifier, reduces its maximum volume before clipping and dynamic range. Cutting the peak is effective but still a compromise compared to a smooth response you get from proper location and tuning.
If you take the time to position and tune properly you set up a firm foundation for the acoustic relationships so that any room correction will have an easy job - and you might not need it after all.
Why are you treating DSP with such disdain? DSP is the perfect tool to flatten out peaks and match your response to a target curve, and anyone with an ounce of knowledge knows that boosting nulls accomplishes nothing. Good automated room corrections will not boost nulls.
Of course DSP should be the last step after you have optimized everything you can control. But in most cases DSP will be necessary, even if you optimize everything else as best you can.
If you have 4 subs in a large room then DSP probably won't be necessary, but a single sub in a small awkwardly shaped room is going to require the use of DSP to get a half decent response.