OK, I reload a lot of 10mm....9mm, not nearly as much. The pressure difference is only 2500psi according to the book. In the real world, who knows what the difference is as the 10 is loaded to max. I don't like taking the chance of that 24lb spring helping the slide seat the bullet deeper causing a pressure issue. Small chance of it happening but there is still a chance. If everything is trimmed the same, crimped the same, then everything is consistent. This also helps when trouble shooting a malfunction as you can rule it out. The 9mm probably doesn't run quite the risk but it's consistent. OCD or just good reloading practice?
I admit, sitting down and trimming 1k cases blows the goat but, I only have to do it once with that particular batch of brass. For me, it's about being consistent with all of my reloads.