My personal focus was that gaining cards might sometimes not be what you want. If you need terminal draw now, that doesn't necessarily mean you need more cards that do it. Ballroom also empties the pile that BoM would leave untouched, which might work for or against it - you can have 15 "Villages" in your deck thanks to BoM, but the same does not apply to Ballroom. Of course, if both are in the same kingdom, Ballroom "steals" them away from BoM, so it's actually an interaction, too. I think BoM has an advantage if you only got to gain it a bit later (with "late" being much earlier here than with Trade Port), or if some cards in the kingdom are good only for a certain timespan or in certain situations. Moneylender, Baron or Coppersmith might be candidates, maybe even the (few) cheap junkers. I mean, sure, i want to play a lot of Sea Hags. But do i want to own a lot of Sea Hags?