No offence taken TheMadScientist, but it really isn't done for ranking purposes but for "Cool URIs don't change [w3.org]" reasons.
You might link to domains, but a lot of people link to pages. If / was currently my sub-niche homepage, and they linked to it because they liked my sub-niche coverage, how would they feel if it later changed to be my niche homepage instead? Bait and switch? (edit: and similar reasoning applies to bookmarks as well.)
Besides, you're not normal. No offence meant, I'm not normal either. Nobody on this forum is normal. I don't know what % of normal takes no notice of the address bar at all, but I wouldn't be surprised if it contained nines ;)