The Tableau data extract engine is a fast proprietary data option and is not a requirement for using the software. You can build all your visualizations against SQL Server databases and have the data served by your SQL Server. The following documentation links and knowledgebase articles will help you understand this option more completely.
Supported data sources - http://kb.tableausoftware.com/articles/knowledgebase/supported-data-sources
Connecting to SQL Server data - http://onlinehelp.tableausoftware.com/v6.1/pro/online/en-us/examples_sqlserver.html