multiple databases on same server

we're planning to consolidate our SAP R/3 databases on a 64bit based cluster running Windows 2003/SQL Server 2005.

the "SAP with Microsoft SQL Server 2005: Best Practices for High Availability, Maximum Performance, and Scalability" document states "SAP products support multiple instances of SQL Server 2005 on the same server. However, running the database of the SAP production system together with other application databases on one server is not recommended." and "An alternative is to run two or three SAP databases on one SQL Server 2005 instance.".

giving this, I'd like to know if it's not recommended even if the 'other application databases' are related to other instances of SAP R/3 product.

it would be useful to know why one SQL Server instance with multiple SAP databases is better than multiple SQL Server instances with one SAP database for each.

thank you.