Perlite is not at all like vermiculite, and does not have the same moisture retention properties. They put it in potting mixtures to improve aeration or drainage or some such thing. For tarantulas, however, there really isn't much of any advantage to putting it in a tank. The only use I have thought of would be for burrowing species, to add texture to the substrate, but even that use would be pretty minimal. Perlite is also very abrasive, due to its sandy texture, so a tank with ONLY perlite as substrate would be extremely unhealthy. Basically, there isn't any significant reason to add it to a substate mix unless you like the appearance of it.
My first batch of potting soil had a sprinkling of pearlite in it, and some of my guys have been on it for ages. Nothing really detrimental, but no real advantage either. I wouldn't go out of my way to include it, for sure.
As far as looks go, I prefer without, because with the pearlite in it, it looks like they're on potting soil, and I prefer a more natural appearance, not that they're lurking around in potted plants, if that makes sense.