Quicksilver responds much faster than Spotlight does, even when Spotlight has "indexed" the disk. I only use Quicksilver for launching applications (i.e. I don't do anything even slightly advanced with it) but it's still satisfying because Quicksilver will find the app as I type it, whereas Spotlight will think for a few seconds.
Quicksilver allows you want to do something other than find and open a file. Though if that is all you want to do quicksilver opens as a default action.
Coming from a linux background I like to automate/do things faster whenever I can so quicksilver is just one way to do that.
