No real downside to them, if you ever think they'll be useful, might as well use them.
Lala, they're a second set of brake levers that attaches to the existing brake cable and allows for braking while you're on the tops.
http://www.performancebike.com/shop/....cfm?sku=18271
Kind of like suicide levers, except they, like, work. Mostly used by cyclo-cross racers but also seems not uncommon on touring and commuting rigs.