I have been experimenting with same rule except much lighter - when one player is left, he has 1min to complete obj or mission failure, with unlimited time. With this rule round length is around 5min ± 2, (occasionally a few min lower or higher). So I imagine that "half team" rule would totally make rounds very short, certainly under 4min (it make sense since you know that at the beginning at the round is the most deaths).
In addition this rule would make the team focus on other teammates (promote cover / support fire etc) since if team looses teammates then mission is kaput.
Of course this is valid only in "Army like" maps / missions and not SWAT like. The most popular maps we have - Pipe and Hospital, would actually in reality be set for SWAT (or Delta Force) and not the Army, so this wouldn't really fly there I guess. It would work in Steamroller because the main task there is to capture territory (which is what Army usually is concerned immediately)
Objectives are set on a map/level basis. Haven't got the Unreal editor on hand right but it should be easy to set a condition such as a mission failure in case half of a team gets killed, if there isn't a viable setting already...i think it would be rather boring tho, if i can say (with unlimited time on a mission such as Steamroller). Also, players of a lower level might make it harder to get through rounds, with that kind of condition (imagine having 3 or 4 in your team, they all get killed...tiny margins for error).