All Data Transformations via SSIS
I have seen this done.  And while it's not something I would ever recommend, it does have some advantages from a process and skillset point-of-view.  The main reason you would try this is to have a consistent drag-and-drop process for building, and importantly, changing ETL processes.

Issues:

1) Performance.  Performing transformations in SSIS data flows is just slower than using TSQL.  

2) Productivity.  Using TSQL is, perhaps, a more difficult skill, but for someone who can do both, it's massively more productive to build and test using TSQL than SSIS data flows.