I've never owned a truck with a lid (cap, topper, shell, canopy, et al), but I've owned a '78 K5 Blazer, and granted with the lid on, everything in the back was pretty much inaccessible unless you climbed in to get it. I've always had a tonneau cover of some fashion for my trucks (whether soft roll-up, or my new folding hard cover), and I like the way they look, in addition to keeping the bed free of debris and dry. If I need to haul something big, I fold the cover out of the way - easy as that.
As a kid, I enjoyed the Hell out of riding in the back of various trucks - with lids, campers, or open bed... made no difference to me. Never had any issues. I think people were better drivers back then, to be honest (less distractions, too). Life was a lot simpler then, and I know I miss it. I've camped out with a truck that had a shell on it - slept in a tent one night, and slept in the truck another. Probably would've been more comfortable in the truck if it was a full-size (I'm 6'5", so an '80 Toyota 4x4 short-bed wasn't the best fit).
As for aesthetics, I think it depends on the combination of truck and which lid is on it. I have a buddy with a maroon Raptor and a color-matched SnugTop... Bad-*** looking rig! Some of the cheaper, old-school style shells don't look right with the newer body styles. But... it's all about what works best for the owner. They definitely have their pros & cons, just like everything.
Why are aesthetics important? They just are for most people. I actually wish I didn't care about them the way yellowboy claims he doesn't. If I were one of those "basic transportation" kind of guys, I would've saved a helluva lot of money over the years driving plain vanilla toads rather than the cool cars & trucks I've owned.