Outside of fights with hard DPS checks (typically not a thing in DF until you can use the echo, and the check stops being that hard) I don't see why anyone would care.

If your void ark run is slow, look to the 15 sleeping DD's, not the stance your two off tanks are using. I guarantee that outside of a parser log, you'll never - ever - notice the difference between a Void Ark run where the OT's are fully using offensive stance and one where they're always in a defensive stance.

If your tank was loosing hate when he should have had it in an offensive stance though, there's again no question as to what he should have done. Keeping enmity, clearly, is more important than a tiny dps increase when no DPS check is present.