Mainstream doctors are not the best source of information on supplements, although I wouldn't go trusting the likes of Dr. Mercola (or even Dr. Oz now that he's gone a little quacky) either. When I consider the uses to which silicon is put (including the making of silicone, which is used for scar sheets and breast implants, among other medical devices), analogizing it to mere sand seems kind of shortsighted.
Here's the link to which I was directed by the Linus Pauling Institute:
http://www.nap.edu/openbook.php?reco...10026&page=502
To get to the detailed information about silicon, go here, or you'll be clicking through for the rest of the day (well, that's how it felt to me):
http://www.nap.edu/openbook.php?reco...10026&page=529
I use Natural Standard for most of my supplement information, but the very long user agreement they had me sign prohibits reposting without permission. I am allowed to send you a PDF of the monograph, though, if you'd like to read it.
I use JarroSil silicon drops. Silicon allegedly curbs the need to smoke--too bad for me that I do not make a good proof of concept for that theory--but I noticed that it does seem to make hair and nails somewhat stronger when used consistently. (Of course, it's also possible that my use of silicon drops coincided with an improvement in my thyroid function so that silicon had nothing to do with it.)
Silicon apparently helps to increase bone mineral density, and back in the days when aluminum was blamed for Alzheimer's, it was used to protect against aluminum accumulation in body tissue. (Of course, now that Herpes Simplex I is the Alzheimer's flavor-of-the-decade (and quite possibly the millennium), the aluminum theory becomes an illustration of how far off track it's possible to go when one is desperate for a solution.)
I didn't actually read the article I linked to (all tuckered out after the clicking through) but I'm sure it covers most of the important facts about silicon.