I'm glad that this feature has been employed. It is definitely a relief to not have to worry about a second gunner joining during the battle.
However, I still find a few issues with how this system treats returning to the lobby. When the gunner is in slots 3-4 before the match begins, when we return to the lobby they auto-switch back to engineer. This creates problems when trying to manage crew roles, because I've already experienced cases of gunners not realizing they were auto-switched to another class by the time the match starts, or there's a mad-dash for a giddy crew member to become the gunner when they ought to not be.
Why doesn't the crew lineup just reshuffle after the match to put the current gunner in slot 2?