I actually liked it when you could search by user name and see what other people bought (long time ago -- it's been quite a while since you could do that). We used to search my Dad, see all the stuff he was buying, then know what we didn't have to get him for Christmas.
But to your question -- I think it's helpful to know what a buyer has been buying, especially when you are selling a high-dollar item. If you click on a bidder's history and see they have bought small items, then suddenly they are buying a car, or a diamond ring? -- well, that's often a sign that the account has been hacked.
It helps me as a seller, too, especially when I'm selling something I'm unfamiliar with. I get a bidder, I click their history, and I see they have bought other similar items -- then I know I've priced the item well enough. I wish you could find out who has been "watching" an auction -- it always bugs me when people watch an auction, but the item doesn't sell -- I'd like to know if some of those watchers buy similar kinds of things, and then I might know if I have overpriced my item.