Why? As long as it doesn't shift from filler to cut it is a good way to "normalize" the volume of powdered substances.
Of course fillers should be practically inert, at least in terms of psychoactivity or interaction.
Fillers have a bad name because unknown compounds in ecstacy tablets are thought to be occasionally unwanted or at least not trusted. Or can you think of another reason?
I've had DOB and 2C-T-7 in capsules from other people that were both perfect, I think there was starch with the 2C-T-7 but the DOB I don't know. Apparently they were both prepared adequately because it was all reliable, starch is not soluble in water is it so they wouldve needed something to grind it good.
But I strongly recommend people don't do this at home because its tricky and without a pharmacy degree or something it shouldn't be done. With substances people react so differently to you don't want 'standard hits' ! Which is why I redissolved both the 2C-T-7 so I could use fractions of the hits.