I don't think it's invalid. Sometimes my carry guns have staggered FMJs and HPs.
Heck the experts thougth the world was flat at one point too.
Valid reasons include light concealment (car door, refrigerator, wood desk, windshield, etc.). Also, the skull of a pitbull is probably pretty tough.
Nobody is suggesting "counting" rounds. But repeated rapid shots will deliver two rounds. One will be more effective. If the HP is the less effective and you only had HPs, then maybe you're unable to effect (penetrate, etc.) the target.
People often gripe that "handguns are underpowered, don't penetrate enough, are only good for fighting your way to your rifle, etc.... Well, then a handgun FMJ that penetrades deeper may be more effective. Shots are not always straight on... a shot to the side may have to penetrate 18" of muscle and bone before being effective (through arm, etc.).
I think it's a valid approach and sometimes do it myself, particularly in my 9x19 carry when I run (risk of dogs, with tough bodies/think muscles and bones, and they don't "know" they are shot), and winter due to heavier clothing.
Any volunteers to stand in front of my "ineffective" FMJs? I didn't think so...
There is no magic bullet, just placement. Some bullets penetrate better. Bullets behave weirdly... some HPs fill with drywall when shot through drywall and act like FMJs. Some HPs don't expand as advertised. Practice often, shoot bullets that your gun reliably feeds, and you'll be fine. As I said, I sometimes stagger mine simply to offer different rounds in the event my targets are varied. And I'm not counting!!!! Simply firing to stop a threat.