I'd say - the more costly the piano, the more elaborate the sheet stand. Cheap piano = block of woodFazioli = not a cheap block of wood
I guess it's true that solid stands (which don't have too many elaborate carvings on them) are better for writing on, as you (i.e. Quantum) and Outin said. though I didn't really think about that. I keep most of my music in binders, books, or, more recently, on my tablet, because I hate having loose music sheets. In fact, to go a little bit off topic, on my digital piano, I have actually lowered the music stand and put a 24-inch 1080p monitor on top, powered by an old netbook, so that I can view my digital sheet music library on two pages at a time. My upright is too big for this, so I just use my tablet, though I do wish I could have a bigger screen. (I have over 1.2 GB of music on there.)Maybe piano manufacturers should start offering music desks with a built-in screen, and maybe a very small computer running a customized version of Android or something like that, for just such a purpose. Pianos are already extremely expensive instruments as it is, so adding something like that would not add a significant amount to the cost. Sure, that would make the desk necessarily solid, but that's a small price to pay.
Anyone could start manufacturing music desks with a screen and chips to replace (or attach to the) original. Maybe there's market for it.