well,there would be lot of useful situations, and i think it definitely functions different to copy+paste (not just quicker, but does different things). for example..
say i have three comp, which is staggered like below

for sake of simplicity, i would want to use time remap to basically hold from 40 for say 30 frames. resulting keyframe would look like below

however, if i were to [copy] the last 3 keyframes and paste, rather then doing what i want it to do, create 3 staggered new keyframes like above, but does below,

copies the three objects and pastes, not the keyframe.
however, my understanding is, if you were to selecte those 3 keyframs from first screenshot, and alt+drag to the right, they should create a duplicate of the selected keyframe, maintaining their offsets between each other, and be moved to position where i let my mouse drag go.
however, currently, what seems to happen is that it just redistributes the keyframe relative to each other (as in does not duplicate, but just rearranges selected keyframe)
this is very useful in it self, but read somewhere that ALT+drag is suppose to duplicate keyframe, and was wondering if it is cs5 chage? or there is different key combination etc etc.