Air carrier departure from DTW voiced concern regarding ATC's use of anticipated separation with regard to successive take offs from DTW and other same type airports.

Narrative

This is not the first time I have seen this happen; especially at DTW. Because of clear weather; the flow of traffic for departure was so steady that don't think we ever stopped while taxiing to the runway. The controller handling our flight cleared a DC-9 flight for departure ahead of us; and immediately cleared us to taxi into position and hold. Up to now; all was well. What bothered both us is that the controller then cleared us for takeoff...and the DC-9 was still on the runway. In fact; I would venture that the -9 continued to roll for at least another 1000 feet. When we were given our clearance; the DC-9 hadn't even begun raised his nose wheel. As I said; DTW Tower does this pretty frequently; and it is in my opinion extremely irresponsible. There are two simple questions that the controller should ask him/herself before doing this: what if the holding aircraft loses sight of the aircraft on the takeoff roll during their own takeoff? Second; what if the first aircraft has to exercise an RTO? ATC needs to be reminded that clearing one plane for takeoff before the other is airborne is reckless and unnecessary; especially at airports like DTW; ATL; and others that have parallel runways and use one for takeoff and another for landing.