Please , could you explain me this excerpt ?
"The exception is tabular models that have been deployed in DirectQuery mode, which do not retrieve data from an Analysis Services data store, but retrieve data directly from a SQL Server data source. You cannot query these models using MDX, but must use a client that supports conversion of DAX expressions to Transact-SQL statements, such as the Power View client."
source : http://msdn.microsoft.com/en-us/library/hh230973.aspx
In the same page , I found
"You cannot use MDX or DMX queries in a database that has been deployed in DirectQuery mode"
which seems ( to me ) a possibly annoying limit to the use of MDX/DMX , so why not to use only DAX ?
Have a nice day