Why is a "Fixed In" attribute missing in out of the box RTC system (including Jazz.net)?

Thats exactly the point.. "The Plan can change while the work item is unresolved".

"And effectively becomes Fixed In when the work item is resolved" - that means you set the "Planned for" only after the job is done.. then how can one rely on this information?

"You can also query for specific resolution (such as Fixed) if you are only interested in workitems that were actually implemented (as opposed to duplicates, won't fixs and so on) " - I'm interested to know when the open issues are planned to be fixed, more than actually looking into the ones that are already fixed..

I'm okay if a particular work item would be fixed only in version 5.0.x, or 6.0.x, but i need to know it to be able to mitigate the issue internally in my company.. How much efforts i can afford to mitigate/find alternate solutions depends on when this would be fixed..

You don't set Planned For after the job is done. You set it upfront. Under special circumstances however, the plan can change during the lifetime of the workitem (like when we decide we won't make it to the next release and need to reschedule etc.) but that's probably more of an exception than a rule. I don't quite get how introducing Fixed In would resolve this issue.