Using SSAS 2005/2008 as PowerPivot Data Source: Query Designer
The query designer in PowerPivot has a few limitations that require the manual modification of the MDX query to get data that are often needed to create a PowerPivot model that compares data from cube with data from other sources (like Excel workbooks or external databases). For example, it is not possible to import the attribute key (the MDX query designer import just the attribute description). These limitations can be overcome by modifying the cube structure, but the point is that a well-designed cube following all the existing best practices results in a hard to use data source for PowerPivot, because it is not easy to import some data that are usually hidden in PivotTables, but important to create a model that relates data from different sources.