One reason I found to have at least a couple of manuals is for a sanity check.
I was working up a load for my 45 ACP using Unique and data from the Lyman manual. The problem was it seemed awfully light on the charge, so after I'd been thinking on it for a bit (while loading a few near the bottom of the range) I pulled out my Speer manual, and noticed that for the same bullet and powder, it was 1-2 grains heavier charge, both starting and max load.
I double checked the load data on the powder manufacturer's site, and found that the Speer manual was matching it, but the Lyman was decidedly light loaded.
When I eventually got those test loads to the range, I found that the Lyman loads would leave the barrel, but would not cycle the slide on my pistol.
Even the first two increments from the Speer (and Alliant...) load data shot reasonably well, and had enough energy to cycle the slide to boot.
This is all just my .02 worth, but I'm definitely a believer in more data being better.