BM/fairgrounds/cost reduction can make for awfully big fairgrounds, picking up non-terminals for free can basically make playing BM read "gain a card". E.g. BM/quarry means that any cantrip 2 is free (e.g. pawn, haven, hamlet) and a lot of the 3s have minimal cost. If you throw in a bridge (say one bought from the BM) or a princess, you can start expecting there to almost always be a very cheap card to snag from your BM. Get a KC or a TR in play and you can reasonably start adding 3-4 unique cards per turn. If you can ever break 5 cards/turn (between buying them from the BM deck and buying them from the kingdom) it actually becomes far more lucrative to buy more diversity than more fairgrounds (up until each fairground is worth 2x the number of fairgrounds you have).
Now this isn't likely, you need some board that just screams massive BM play and some way to slow the game down enough to prevent a colony engine from destroying you. Such boards are going to be rare, but I could conceivably see it happening. Something like library/quarry/BM/village/TR/fairgrounds might be enough to encourage folks to split the fairgrounds & never 3 pile because each player is gaining 8 points a turn in fairgrounds value.
