I’ve always assumed it was there for slower machines. That is, if it takes a while for results to come back, you might want it to wait until you’re done typing to start the search instead of cranking away on stuff you don’t want. Maybe this isn’t as much of an issue as it was in 2003. :)
With my machine being significantly slower (Since upgrading to Lion) I am actually seeing a requirement for this setting now.
I use a value of about 0.05s, which seems to help significantly with loading objects, so I suggest we change the QSDefaults value from 0.1s to 0.05s. I will issue a pull request soon if I hear no objections.