The Perlite also displaces moisture, thus reducing the overall water-holding capacity of the mix.
It holds moisture on its external surface pores, but not internally.

You are correct that fine bark dust can be used as a peat substitute in many cases. I do so with mixes
for my containerized ferns in particular. I have also used a bark/perlite medium for collecting
Maple seedlings and growing them for a single season before potting on to a more permanent mix.

The issue with an all-bark mix is moisture retention, I believe.
100% bark, like 100% peat, will support a perched water table in a container. I also think
that the bark will dry more readily in the top layers, well before the lowest layers need to be watered again.
I love bark, and I love working with bark, but it needs to be amended for a balanced mix.

Josh

I would agree with Josh. I, myself, use a mixture of fir bark, coarse perlite, and granite chips. It's more along the lines of the Gritty Mix than the 511, but the concept is still just about the same with regard to aeration and drainage, and us being more in control of the feeding and watering needs of the containerized planting. Size of materials being fairly comparable is what I go for... unless it's a planting that requires more moisture retained for longer, at which point I add a bit of high quality potting mix or something similar, still keeping the majority of materials used of larger size than fine silt/peat.

Don't let me confuse you, though... keep in mind that growing in containers differs greatly from growing in the ground... and that we each have our own individual environment to consider when planting anything. There's really a lot of great material to read here in the forum... and I think you'll find that it's the concept, the how and why of it, that's really important.

Many nurseries use precisely that--a 100% pine bark mix--with great success, especially for containerized trees and shrubs. Much has to do with the characteristics of your bark, e.g. particle size, degree of decomposition, etc. I like the peat component because it tends to retain moisture (especially important if you're using fresh pine bark) and provides fibrous strands that hold the mix together. The perlite then balances the addition of the peat. This year I'm trying an 8-1-1-1 mix of pine bark, high-quality peat based potting soil, perlite and Turface. As Greenman and jodik pointed out, it's the concept that's of primary importance, rather than any exact "recipe."

you can use 100% PBF with great effect (see below link).

PBF dust will act similar to peat by adding moisture retention and raise the PWT.

Peat might also have a lower pH than PBF, but i'm not sure the pH PBF dust only compared with peat.

https://projects.ncsu.edu/project/woodsubstrates/documents/research/comparison-pine-bark-vs-chip.pdf