The problem with not mirroring (and ideally then performing the forensics on the mirror) is first that if you perform destructive testing the other side can't verify your data, and second that there is no way of establishing that even nondestructive testing didn't alter or destroy data from its original state.

But if no mirroring was possible due to the deliberate acts of the party objecting to the test results, for example in an effort to destroy evidence in a potential civil or criminal case, I doubt that a judge would exclude the evidence on that basis alone.

Some companies do amazing recovery work, even with severely damaged drives - Examples.