Interesting conversation, as I just started to sell my own shapes, I had those exact thoughts. So, my point of view: there is a demand for shapes, not all people like to spend time to tweak the sliders, some want just some look without the time investment. Me, on the contrary, making cool looks is a major fun part of SL for me, I don't ever stick to any specific look. It's not only SL, I liked making pretty sims and always spent hours in character creator in all kinds of online games. So for me it's like a hobby that may bring me some small profit....so I can buy stuff to make some more cool shapes, lol.
Also the shape is not only the numbers in shape editor, it's a character. The combination of hair, clothes, etc. brings some added value, I see a lot of great avatars in SL and I often wonder where did they get the cool stuff/skins that they are wearing.
And yes, about the profit. I do think that shapes are easier to make than other stuff, it doesn't require any tools or special skills, just a good eye, so I sell mine for only 99L.