At first glance it seemed surprising it would have even been offered with the BB. I could be wrong, but for non performance use, wouldn't the PG be fine? In fact, with the big torque of the 396, I bet you really wouldn't even notice much of a difference through the range. The PG has a tallish 1st, but the final ratio is the same as the TH. Another plus is that glide is pretty durable. Could it have been a thought towards sticker shaving for those who weren't concerned? GM trying to move an overabundance of now unloved glides? Could you get it with a 427?

I'd say with the smaller low torque V8's in the big car, that's when the TH350 or 400 would prove a noticeable benefit. 

My 66 396 was a Powerglide equipped car.. Apparently the ones that went behind the Big Blocks were a heavier one than ordinary with more clutch packs so might in themselves be worth saving.

My car was a very well equipped car so cost probably wasn't the reason.  The only thing that I can figure (and I don't mean to be sexist here) is that I was told that dealer had bought this car for his wife so she may have wanted something "easier" to drive.

That may have been the case and the reason they still offered it. The 396/325 wasn't really considered a muscle engine (and price wise was quite a cheap option actually) but may have in many cases been part of a car that was designed to be an easy driving, almost luxury car that had "passing power".  Ie a "lady's hot rod".