It's not just the aesthetics in a sense, when they move away from only offering the reference model they build in value adds, like the non-reference cooler which is usually very, very good. In addition to "factory qualified" overclocks which, may not be inherently impressive to enthusiasts, are something to consider for the more mainstream-but-still-interested-in-gaming end user.
So it's not like they only add RGB/aesthetics to the gaming branded cards. For example Gigabyte has had a "Windforce" model around for a while in many of their past releases which may/may not have had illuminated branding(RGB or single diode lit) and slightly more subdued styling. There's something for everyone.
It's a fad to have RGB, matching sleeved cables and components etc... in my opinion.
Not 100% following the thought process here, people inherently like things to revolve around some kind of centralized theme and it gives them the sense of having added their personal touch. Sure, I don't care about internal LEDs, sleeving cables, etc, in server builds(mostly just cable management). I wouldn't really say it's a fad anymore than it could be a "fad" to have kitchen cabinets, countertops, drawer handles, etc match and complement each other. It's sort of like art in a way. If anything's a fad really I would just say it's the adding RGB to everything, but even then, I'm hard pressed to really see why. It's just the 'natural evolution', or direction, of light emitting consumer electronics et al.