Well, Big Dog has managed to set the bar as far as safe progressive loading by positioning himself so he can see inside of EVERY EFFEN case!! I wasn't aware for certain this was possible, but it's worth the effort to catch that ONE squib, or worse yet a double load.
Being a crusty old fart, I continue to charge 50 or 100 cases in a block, and then wheel my chair under the overhead light and peer down every case to see if they're all the same. I have used some powders that did not meter well (thankfully they're now obsolete) but you still have to be on your guard for mystery objects in the powder. Ladybugs that flew into the measure or canister can definitely "BUGGER UP"

your powder measuring, and I'm mot so sure some medium sized dust balls in the powder can't do the same. For whatever reason, you can have an odd charge just decide to rear its ugly head. With ball powders the chance is smaller, but you can't always use those, and depending on the flake size and metering opening in the measure, a very occasional powder bridge could occur, and there's always static electricity potential more in the winter than the summer.
Bottom line - you ain't sure about the uniformity of your powder charges unless you look at ALL of them to make sure some funky 1 time in 1,000 event didn't happen. Yes, it's a PITA, but much less so than a rapid fire mag dump with a squib halfway down the stack.