Under the SQL Server 2012 licencing terms running a data warehouse on the BI edition of SQL Server did not make sense if your source databases were licensed per core.

This was due to the multiplexing rule which, in effect stated that all users who either added data or read from a database had to be covered by a CAL if that database was on a BI edition SQL Server, regardless of how their data ended up in, or was extracted from the BI edition database.

This was particularly annoying if you ran a website and your web databases were on servers licensed per-core and you then ran an ETL or batch process to put data into a data warehouse which only a small fixed number of users would run reports from.

In effect you had two choices, build your data warehouse on Standard or Enterprise editions licensed per-core, or purchase a CAL for every user of your website who created data that would pass through the ETL process and end up in your data warehouse.

Under SQL Server 2014 the multiplexing rule appears to have been relaxed and the new licencing datasheet states:

Business Intelligence Server Access 

Similar to other SQL Server products offered under the Server + CAL licensing model, Business Intelligence (BI) Edition generally requires a SQL Server CAL for each user or device accessing the server software. New with SQL Server 2014, use terms for BI Edition server software now allow batch processing of data without requiring CALs for those data sources supplying the data. 

• ‘Batch Processing’ is defined as an activity that allows a group of tasks occurring at different times to be processed together at the same time. 
• Non-batch processing access to BI Edition servers still requires CALs be assigned to those users and/or devices accessing the server software. 
• The general multiplexing policy still applies to SQL Server Standard and Enterprise Edition software licensed under the Server + CAL model.