While I can sort of see where you are coming from in regards to the design, I don`t agree with the latter portion of your statement. BTTF is one of the Sci-Fi movies that has defined science fiction as we know it today. Many, many films stream from if and other movies of the time period, SW, Aliens, Blade Runner, etc. I only mention this because it puts into perspective the scale of the fan base that has been created over the last 30+ years. It wouldn`t be much a stretch to say that you`ve (generally) been living under a rock if you haven`t at least heard of BTTF in some context. Combine this with the numerous Lego fans out there and you have an item with significant demand. Obviously, I don`t work for Lego and have not seen the official sales numbers, but even though 21103 doesn`t appear to be on the scale of Minecraft, it looks to be selling quite well (I base this on the availability of the set online, as well as it constantly being in the "Top 25 Best sellers" list on Lego.com/ca, until recently).
That alone gives the set an appeal, if not for build purposes, for nostalgic, fanboy reasons. So yes, I get that some may not feel that it is a set with much "flare" or "appeal" as you put it in terms of design, but it does the job, and it excels IMHO. Granted it isn`t perfect, there are still some changes/alterations that may have looked/functioned better, but the 3 in 1 options for the set were done quite well overall.