There are many advantages of leaving the data within that tool that owns it and providing a link to it:
- The tool that owns it knows best the rules to govern changes to it, including auditing support
- The tool that owns it, knows best how to control access to data. By copying the data, often access controls needs to be replicated as well (as best possible)
- State-models of resources across tools often don't align
- There are ways to expose this data in other tools, without replicating it
Update: Also see interesting and related IBM DeveloperWorks article "Stop copying, start linking"