This would ensure two things that are not true today:
1) A command that unstashed locks held by a transaction waits for writeConcern of the system last OpTime. Today, because the locks are stashed before waiting for writeConcern, the command may end up waiting for an OpTime in a previous term.
2) Stepdown will kill a command that is waiting for writeConcern after having unstashed and stashed a transaction's locks. The command may be waiting for writeConcern even though it did not do a write, because its Client's last OpTime was set to the system last OpTime.