If you check BeiJing SID you will know we would like aircrafr dep from middle runway to maintain TRACK for 2 minutes at least ,currently we are using the RNAV for departure ,there is one waypoint that is only one minute after departure but we have found that pilots may made a mistake and turn to wrong place ,so we like to let aircraft maintain runway TRACK for 2 minutes and then vector join SID.
As you said aircraft will drift same even if they are side by side ,it seems right ,but what if aircrafts are in different layer ? How to make sure that the wind is still the same ? Normally aircraft from the east runway will turn heading 030 after maintain runway heading and climb to 900 ft ,they said it is their airlines' rule ,and if the middle runway departure aircraft drift to the upwind of east runway ,that will be a little bit dangerous.
I am not overthinking ,these did happen for several times ,we tried to let aircraft maintain runway track ,that recude the work load of departure ATC a lot .