I am big on side by side comparisons and do a lot of them. However I do them with very specific goals. I think you are right in that side by side comparisons will show you something you otherwise can't see. For instance I know I want to use Acros, so first I do density tests with the developers I am interested in so that I can get the development exactly as I would use it. I already know the developers I want to consider. Then I set or find a situation and shoot a couple of rolls that I can cut in the dark and develop in the different developers. Then I make prints and mark them on the back in pencil. After they are dry and I don't know which are which I study them until I see differences. Then I look at the back and reveal what they are and then I study them again.
It is useful to me to do this because I have very limited variables. If you wanted to compare a few films then it would be useful to already know what developer you want to use.
For instance I knew I wanted to use Beutlers developer for a project and I needed to use a 400 speed film. So I got several 400 speed films and figured out the correct exposure and development times for all and then did a side by side comparison on several situations/subjects. I was able to learn which had the most acceptable grain and which had the best highlight detail and which had the higher or lower mid range tones.
The problem most people are pointing out here is that there are too many variables with several films and several developers. I agree with that.