Same here, it seems to hold moisture better when mixed in for species like my haplos.I use vermiculite, a fine grain one, only for help mantain the humidity level for those T's that needs humidity on a higher side (which obviously isn't the case of a P.murinus of course) like H.gigas, M.robustum, E.murinus and so forth.
I just add an "hint" of that in the substrate when I make, days prior, the set up for certain T's. Works pretty well for me. I have no clue about the more bigger, rough one, on the other hand.
It's an additive basically and not IMO something to be used as "substrate".