The diamond is much maligned, I've even heard pickers I respect call it dated or even obsolete. But it's much more useful than some of the other picks that get thrown in to fill out pick sets. I'm looking at you Frosty the Snowman!!!
The beauty of the diamond as Schuyler Towne pointed out is that it's halfway between a pick and a rake. While there a picks and rakes that do their jobs better, the diamond is a nice convenient middle ground. The Peterson Gem is basically a hook finished like a diamond and it's popular because it works.
I recently started to go back through my locks and use the picks I don't normally reach for. Instead of grabbing a hook, Gem or Bogota, I force myself to use the diamond, L-rake, s-rake, etc.. When I use the diamond as a rake/hook hybrid it's incredibly fast on some locks.
The L-rake (which ISN'T used like a rake) works almost as well as a key on some locks. The S-rake? I'll use the diamond instead. But I found that the picks I neglected work well and better on some types of locks than the Go-To picks, So the diamond moved a few placed up in the kit.
I have never owned a Snowman, so I can't honestly say it's as useless I as think it is. Maybe for wafers, but if I owned it I'd be it's Professor Hinkle