Just trying to get my head around something :
What is the advantage, or reason why someone would use an SSAS datasource for PowerPivot?
You never know what the end-users are trying to get in their heads around. :-)
Using SSAS as a sole datasource could be useful if you want to add own calculations and KPI's in your reports/Analysis instead of waiting for IT to add this to the Enterprise cube.
Using SSAS together with other data sources could be useful to enrich the data of the Enterprise cube.
As far as I know isn't there any possibility to Export SSAS Tabular into an Excel file (like the other way round through restore in SSMS and Import through SSDT).
Not everything Technical possible is useful from a Technical perspective of view, but can make sense for the individual end-user. In my opinion the Enterprise DWH/cube should deliver everything on the Long run. On the short it makes the end-user's life easier if (s)he can fix it on her/his own.