HOTSPOT
You are developing a SQL Server Analysis Services (SSAS) tabular project. The
DirectQuery Mode property of the data model has been set to On. The current partition for
the table is configured as the DirectQuery partition.
Data is loaded into a data mart throughout the day and must be available in the data model
as soon as possible. The model must support querying by clients that issue both Data
Analysis Expressions (DAX) queries and Multidimensional Expressions (MDX) queries.
Clients issuing DAX queries must receive real-time query results.
You need to select the appropriate Query Mode property for deployment.
Which query mode should you select?
To answer, select the appropriate query mode from the drop-down list in the dialog box.
Answer: 
Why not direct query? Performance is not a consideration here, so I would think just DirectQuery.
Ok, so answer to my previous questions is because of following:
A. If we are partitioning and select one partition for direct query but don’t expect the entire model to be direct query, we have to use direct query with in memory.
B. MDX functionality is limited with DirectQuery.