Too many posts!
Not sure, but it was commonly used in palynology. I'm not sure why. Maybe it took fine deposits off the grains so you could get a really sharp image.Don't know.
We used it to dissolve alumosilicates but we were testing at some point if we could go to aqua regia directly adding at the same time a little bit of boric acid, thus bypassing the hydrofluoric stage. Seemed to work to some extent, I think some types of rocks (we mainly worked on endogene) dissolved better than others.
Hydrofluoric itself has some problems because it can form stable tetrafluorates which are never going to dissolve no matter what you use. You had to get the concentration/amount just right and I guess that went for boric as well.
I wouldn't imagine palynology needs to deal with a lot of alumosilicates because spores can't survive in say anything of higher grade than regional metamorphism where you start forming feldspars, which are the dissolution nightmare. Maybe you can find spores in something like a gneiss or similar but then again, I imagine you wouldn't care if the feldspars dissolved or not since your spores wouldn't be in there.