I don't like how gun advocates often describe assault weapons as just "looking" like assault rifles. For the most part, these aren't cosmetic (or as you describe, "aesthetic") differences, they're ergonomic differences. That collapsible/retracting stock is designed for greater maneuverability in tight quarters while still allowing steady firing for longer distances -- it's not merely a fashion accessory. The pistol grip allows the rifle to be overall shorter and easier to maneuver in tight spaces (say, inside buildings). The detachable mag allows for quicker reloading.
It's not a coincidence that assault rifles often have these same design features and the advantages they bring don't vanish just because the gun can only fire in semi auto -- in fact, trained soldiers/officers using fully automatic assault rifles will use semi auto fire most of the time anyway (contrary to how they're depicted in movies/video games).
edit: This is a perfectly legal civilian weapon (semi auto ar-15):
I also think it's worth pointing out that semi-auto rifles can still fire quite fast:
https://youtu.be/TuNSB9jkpyQ?t=1m32s