Reporting services is a great tool. Report Model is the best features in the Reporting services but the biggest problem with report model (Data source view) you can use only "Inner Join". you can not use outer/full/right/left joins.
This is a biggest drawback of the report model. I hope they will fixed this issue as hot fix. Many users and communinty complaints about that. If this issue will fix in near future then lots of new customer will like to have this product in their environment.
I am sure you heard about this issue, if not please search on google so many post out there, where you will find users and developer are not happy with this drawback.
If there is way to use outer/left/right/full join then please post a update at microsoft technet or msdn website.