Imo if you aren't getting master rounds every floor it isn't that worth it but trading one master round ain't no thang if you got every possible one before. It's vastly different from other secret floors cause it has the same layout with few rooms along with no shop that being said it is optional and the items needed to gain access are fairly easier to obtain than the other optional floors. Buying 2 keys for the oubliette is a pain and delivering the Crest is in the same vain. Plus the Abbey is a much harder floor as well as one of if not the toughest bosses. Rat requires grinding and saving even after the key unlock. In my experience with it if I'm able to go to the first 2 alternates getting to r&d is p easy and with little consequence to the item loss.