Hi Daniel,
BW has always been a part of Netweaver platform. Now, with Hana, I assume that CPU impact for BI activities is now acceptable for ECC activities. Both activities can run on the same DB without impairing each other. The interesting point is that no replication is needed, your BI reports are directly executed on your ECC tables.
So, if your BI reports are only built on top of ECC data and if your BI users have an acceptable CPU consumption, your assumption is right, no need for an additionnal box.
But, if you need to build an Enterprise Datawarehouse with data coming from multiple systems, then an additionnal box is probably the best architecture.
Regards,
Frederic