The only reason I can think of as to why the Discovery has such long Nacelles on the rear was for easy testing of different Warp Field interactions with different Warp Nacelles without having to worry about Line-Of Sight clearance based on existing Vessel Structure getting in the way. Usually you have to figure out some way for the Warp Fields to meet when structure is close by, but with the Nacelles sitting so far back away from the StarDrive section, it makes for easy configuration / testing. I see the Crossfield class as a giant R&D / Test platform. That's what makes the most sense to me. Given the name Crossfield was taken from Albert Scott Crossfield who was a test pilot, it wouldn't surprise me if the entire Crossfield class's design / simple shapes was for being the giant R&D / Test platform. Also, they did mention in show that Discovery had a very impressive number of scientific experiments happening simultaneously.