I think the most commonly cited reason for dropped seatstays, at least by the manufacturers, is that it allows more flex/compliant ride qualities. I don't necessarily buy that reason, as any flex in a triangle(either the rear or front) seems to be more related to tube shapes and carbon layups, rather than the length of the sides...but I'm not an engineer, so I could be wrong. Also with the prevalence of disc brakes, they don't have to have the chain stays in traditional spot.

Me personally, my main road bikes(Canyon Endurace and Ultimate) have more of a sub-compact geometry(slightly sloped top tube) and don't have much in the way of aero concessions. The seat stays meet the top tube just in front of the seat tube, so not really dropped stay IMO. I don't really like the look of dropped stays anyway, so either bike looks "modern," but also has enough of a traditional "bike look" for me. The Endurace is a somewhat racy endurance frame and the Ultimate is a race bike/climbing bike. The frames look very similar to each other, but the Endurace has a smoother ride, which I attribute to a different carbon lay up and a springy seat post. The Ultimate isn't as smooth, but does an amazing job of transferring power to the wheel...it just feels fast/efficient when pedaling. It's just an example of how a very similar frame design can be altered with different carbon lay ups to have much different ride qualities.