It's lost lots of value these days, due in part to racquets. It's still very effective in the amateurs and as a change of pace in the pros but in the old days with the old racquets, there was a much lower chance that a player would take a decent slice and rip it to take instant control of the point. It's a much great possibility these days though it probably doesn't happen as often as players think. I think it's a bit like the volley - it's clearly not as useful as it was in the old days but it's still a lot more useful than people think. Players just don't know how to use a slice anymore other than for defensive purposes. I do mean "use" because yes, players can hit a decent slice, but they don't know the nuances of when to deploy the slice.
For me as a 4.5 amateur (once 5.0 but age does me no favors), I find the slice extremely useful because at this level, most guys either over hit or do nothing when they see a slice coming at them.