It´s prerequisite for issue 4197 to be effective.

• Can be implemented only in some plugin for 4197 (store copied jUnit results
  separeted for complete history of job)
• But there is other way, that can be usefull in more ways.

When you discard old builds, function form 4197 is uselles. You need the jUnit
reports history. But on the other side, you need the discard old builds with
parameters (max X days, max Y number of builds) because of disc space.

My idea is some sort of "list of checkboxes" that selects what to discard/keep.

Possible use case:
Set treshold to 25 builds, discard old JAR/WAR (200MB each build), keep jUnit +
cobertura results (very low disk consumption)

But it is more complicated way because many other Plugins must be upgraded to
add themselves to this list.