Hi Axel,
SSIS is very powerful once you get over the teething problems. If
you're used to DTS then it can be quite frustrating at first, but once
you get over the differences it is a vastly superior ETL tool.
Data Sources are essential for SSAS (and I believe SSRS as well), but
it seems MS had a different idea of how they should work with SSIS (or
maybe they just never got around to implimenting them properly

).
Either way, I certainly don't use them for SSIS as they make things
more difficult without actually adding any value.
Good luck!
J