A few days ago, Microsoft released a new official whitepaper (“How to cluster Analysis services”) regarding Analysis Services and Clusters. As you probably know the Analysis service used in SharePoint for PowerPivot “SharePoint Mode” (released with SQL Server 2008 R1 and working since SharePoint 2010) is a special engine and is kind of the… ancestor of the new Tabular Mode. (Actually it is a tabular engine)
This is the first official statement regarding this aspect.
Here is the extract that matters the most:
<<Clustering Analysis Services in SharePoint integrated mode (PowerPivot for SharePoint) is not supported, nor is it necessary. Availability and scalability are built into SharePoint, with SharePoint providing load balancing among server applications in the same environment.
SharePoint 2013 farms that utilize external Analysis Services instances (configured for SharePoint integrated mode) do not support clustering the SSAS instance. If you need scale or redundancy for this scenario, install multiple standalone instances of Analysis Services in SharePoint mode and register all of them in Excel Services configuration pages. Excel Services will automatically load balance the requests among all available Analysis Services servers. Conversely, if the PowerPivot workloads are small across multiple SharePoint farms, you can use a single Analysis Services instance for both SharePoint farms.>>