Some people prefer to climb hills in the drops - look at old vids of Pantani, RIP (good example at about 3mins 30seconds http://www.youtube.com/watch?v=ZVmsYp6MkCE ) and Ullrich.

I got to ride Pantani's bike once, and I found he had his bars surprisingly high, so his drops were nearly as high as my lever hoods. A good height for climbing.

The other time it's necessary to use the drops is really bumpy descents. Cobbled descents especially can make your hands drop off the hoods, tops of bars. When you're down in the drops your hands can slip off.

Edit: but I guess you can't reach your brake lever when you're in the drops - bad for descents