The full scan of say, a production database server's VHD negatively impacted performance to an unacceptable degree. It's something that probably could have been fixed with a more powerful storage subsystem.

RCT not only gives the change list, but I believe it also ensures that on a cluster shared volume (CSV) there is an easy way to safely get the changed blocks. Normally a VSS operation on a cluster shared volume puts the drive into redirected mode, in which all IO has to go through the host that owns the CSV at that time. That also negatively affects performance, effectively limiting throughput to whatever the network speed is instead of the usually much faster SAS bus.