a tiny bit of coco coir to fluff up the gaps can make certain trees easier to care for and help hold everything together.
Eric Shrader from Boinsaify uses coarse perlite and coco peat/coco coir for development stage and pre bonsai, with a much higher % of coarse perlite in the mix and coco coir to help fill the gaps a bit and lift CEC, and make the soil 'grip' younger trunks better.
I mix it together with the majority inorganics in a bucket with water, then scoop it out with a fine mesh colander, press it a bit and put it out to dry in the sun before potting so it expands into the gaps and makes the mix more homogeneous. So far, good results from using it in air layer bags, post-air layer up-potting, rooting large cuttings, getting rooted cuttings into training pots, filling out nursery cans etc. A cheap brick goes a long way as well and is easier to store than bags of organic potting soil. I would never use a majority coir mix though.
It's good for something really thirsty like a ficus as well to help form a mossed core root ball that holds itself together when placed on top of a shallow pot filled with 100% bonsai mix to make a hill. I'm chopping this root ball+potbound shallowball in half this summer and repotting into same pot so it sits lower though. Without the coir, I would probably have to water this 3-4 times a day in the summer....
