It doesn't make sense to me that humectants would be bad for porous hair.  I think of very porous hair as kind of more fragile than less porous hair, but I don't know exactly the definition of it (I picture hair with little "pores" or tiny holes in it, like the outside of each strand might resemble swiss cheese).  So it seems to me as though humectants would be good, to protect the hair from damage.  My lay person impression is that very porous hair will soak up water and take longer to dry than less porous hair.  But water and humectants are kind of two different things.