Why should somebody use BI after implementing SAP? What are the known/unknown drawbacks of SAP that BI will be able to address effectively? For instance, why should one go for ETL tools like Datastage or the Datastage load PACKS of Ascential software after implementing SAPa dn -- how would it help?
Are you asking why someone needs Business Intelligence (BI) after implementing SAP (R/3)? Or you are asking if someone has implemented SAP Business Intelligence (BI) Solution (using BW) then why you need 3rd party ETL tools (data Stage etc)?
The reason for using non-SAP ETL tools, like those from Ascetical, is that BW does great job to extract data from SAP R/3 but has no Extraction capabilities for non-SAP data sources unless you build a flat file that BW can read to load data or to use 3rd party tool for complete non-SAP data sources ETL integration from within BW.
Similarly, SAP BW has no true data extraction capability. Here you can also use 3rd party ETL tools to extract data from BW for other applications or data marts. SAP is planning to deliver Hub and Spoke functionality in BW 3.0B release that will enable you to extract data from BW. I am not sure if Hub and Spoke functionality will be an add-on product that requires a separate license or will be part of BW 3.0B.