Erik Mona said:
I am considering keeping it this way, and I guess I'm looking for suggestions from the faithful as to why that might not be such a good idea.
The fact that it'd fly in the face of what
Planescape people remember is one reason. Another is that the idea of most evil creatures being larva across the planes made a sense of its own, in regards to how the "soul trade" worked (mostly in regards to night hags et al).
Basically, if all CE souls already appeared as manes, then promoting fiends wouldn't ever need larva from The Gray Waste; why waste their gold to buy larva from there, import it home, and then expend effort to make it into a manes, when there are perfectly good manes right in the Abyss? This then undercuts the entire idea that there need to be "larva herders" (night hags) who then sell the larva to be shaped into higher-powered fiends.
As the "Pox of the Planes" article stated, having larva appear in all the lower planes (as an aside, larva on each plane has that plane's alignment. E.g. larva in the Abyss is CE) creates a need for NE larva. That may sound strange, but the idea here is that CE larva don't hold their forms well - a lot of them are chaotic enough that they devolve back into being larva once promoted (and likewise, LE larva tend to be harder to change, having their own ethos of law). But NE larva accept the imposition of CE or LE better, having no ingrained concepts of the alignments that need changing.
All of that, in turn, makes things more interesting among the Abyssal petitioners. You can then determine who is more likely to become a better demon (they appear as manes); you can have nalfeshnees who, for some reason, decide to send a demon lord they hate the souls of those who're weak (CE larva they promoted to manes, knowing they'll be less useful), etc.
The PS system makes it work for the Lower Planes as a whole, unlike the 1E system.