If the evidence they're hearing on the phone is not admissible, there's a good chance it's because they're listening through illegitimate means as well, hmm?
The main incentive for paralleling something is to protect the source, to maintain its effectiveness.
Parallel Reconstruction is used to avoid scrutiny of the original source of evidence. Even when used to legitimate ends (and I have doubts how often it's used as such), there are fundamental problems with dodging accountability.