And, what's even better, you can leverage both strategies in the HANA EDW (BW on HANA) architecture. No matter which data acquisition / modeling paradigm you use (you're gonna use whatever fits better the business requirements), you can leverage the virtual modeling layer and user authorizations from BW which, in runtime, add no overhead and simplify the maintenance of query definitions and user authorizations to a single place. This document discusses technically some of the possible integration scenarios: SAP HANA Live & SAP BW Data Integration - A Case Study
A very classical case: an typical plan x actuals comparison report, but for which your business area requires real time updates of the actuals. You can have your master data coming from ERP into a InfoObject in BW for better data governance, the planned data coming from a BPC cube in BW and the actuals coming from a HANA Live view. You can combine the 3 sources in a Composite provider in BW and consume it through any visualization tool (e.g. BO) on top of a BEx query built over the Composite Provider.
BW on HANA is about giving options, and to be able to model whatever requirement you have in the same platform is exactly what HANA EDW is about. Another good reading: Blog: The HANA EDW | SAP HANA. Albeit a bit outdated, the concept and future direction remains fresh.