When 5e was being playtested, the proficiency bonus wasn’t a static +2, +3, +4, +5, or +6. It was a d4, d6, d8, d10 or d12. The character sheet was much simpler, too. You had your basic ability modifiers and then you would just note if you were proficient. So if you had +3 strength, you’d roll d20+3 if you weren’t proficient but if you were, you’d also toss on a d4 (at lower levels, and more at higher levels).
This is awesome.
It’s awesome because it makes the math of the game clearer, leveling up more tangible, and character sheets could be made simpler. There was also a faux benefit. “Other players could see if someone was using the proficiency die when they shouldn’t, or forgetting to use it when they should use it, and help correct them.” Yeah, but they wouldn’t be making that mistake if proficiency dice weren’t in use. Players see their sword bonus, like “+3”, and add that. They don’t go “hmm I have +1 dex and a +2 proficiency bonus so let’s add that lol”. They just see the +3 and don’t nerd out about the underlying math. Althoooough. At leveling. Yeah. That’s when this’d be useful. “Hey, you’re level five now, time to roll a d6 instead. So yeah, it’s awesome.
And I opposed it.
Because I, even back then, especially back then, was obsessed with speed and with the tens-complement variant I liked, i.e. with a roll-over (and by roll-over I mean equal-or-higher, not strictly-higher) number that persists statically through the entire fight, and the proficiency dice messed with that. I don’t like “math after the dice hit the table” and the proficiency dice made that math even less possible to, uh, “move outside the loop” to use a programmer’s turn of phrase.
Maybe I still feel that way today; my stance has softened a little bit since bardic inspo and bless can be fun. As long as you don’t apply them post-hoc…
I’ve noticed that this variant is seeing a small resurgence in the 5e community. That’s something I follow with interest and curiousity, I’m glad people are trying it out. Maybe it is good after all.