Barr is fine with Mueller testifying because he believes he can continue to block the release of the full, unredacted report. As long as that’s the status quo, Mueller won’t answer any questions about the redacted parts. Which means his testimony is far less valuable than it would be otherwise.
At least he might answer a few questions about the DOJ policy on indicting sitting Presidents. That’s about the only value I can see in his testimony at this point. It would be very different if the House had the full report, even if it would mean that some of the hearing with Mueller would have to be behind closed doors.