Air Carrier departure from PHX expressed concern regarding ATC's practice of clearing a subsequent departure for takeoff prior to the first being airborne; citing potential safety concerns.

Narrative

Our flight was cleared for takeoff 25R PHX. I was the flying pilot. Before the Captain called V-1; the aircraft behind us was also cleared for takeoff on 25R. I don't understand how ATC can do this. I realize that ATC is trying to handle/move as many aircraft as quickly as they can; but wouldn't waiting until we were at least off the ground before clearing another aircraft for takeoff on the same runway improve the margin of safety? I realize that 25R in KPHX is 10;300' long; and; in reality by the time the following aircraft actually started his takeoff roll there was probably 1-2 miles horizontal distance between us and him; but that sure doesn't leave much of a margin for error; at least not in my mind. If for whatever reason we had needed to reject our takeoff; then; initially at least; there would have been another aircraft on takeoff roll right behind us.... on the same runway! (As a side note: It seems to be SOP in PHX to advise if you have the departing aircraft ahead of you in sight; and if; when you do; get cleared for takeoff sooner than you would at most other major airports. I can't think of another major US airport that does that on a regular basis).