... in FOTR when Gollum was tortured there. And I'm not sure if this short moment when the Witch King get his armor in ROTK wasn't Barad-Dûr too (perhaps it was Minas Morgul). You can imagine how it would've looked like. If you look at Isengard or the Tower of Cirith Ungol. All where like small brothers of Barad-Dûr. But I think that showing the black fortress more from the inside would've destroy the mystery about/around Sauron. Sometimes a glimpse is more effective than showing too much of something especially of bad guy places and backgrounds.
In Reply To
We could have at least been treated to the inhabitants and the colossal armies being built there and then unleashed on Middle -Earth
But we already saw this in FOTR and TTT in Isengard. I'm sure there is no difference in Mordor by creating armies of Orcs and their armor and weapons ;-) "There is only one Lord of the Ring, only one who can bend it to his will. And he does not share power."