The customer for whom the ActiveControl / ChaRM integration was originally developed had a few contributing reasons for wanting to run both tools in parallel:
1) They had previously had some ChaRM Retrofit overwriting Project change resulting in a loss of work. This was their main reason for wanting to implement ActiveControl, so that the Merge functionality could be used instead of Retrofit to keep their BAU and Project landscapes in sync.
2) They wanted to benefit from ShiftLeft analysis checks to improve their existing process and reduce the number of issue during delivery of SAP change.
3) They are a highly-regulated pharmaceutical company; as such, they did not want to completely replace ChaRM with ActiveControl as it would have meant a lot of process rework, internal GXP re-validation and user re-training.
As a result of the ActiveControl / ChaRM Integration, the customer now deploys transports through BAU track via ChaRM, and use ActiveControl to manage and deploy transports in their N+1 and N+2 tracks (and for Merging between tracks).