My opinion on the subject is that executing a takedown (unarmed or not) would take a lot of time,but would be more silent than firing your unsuppressed firearm. Its not 100% silent,but its better than having the enemy combatants within a 2 kilometer radius hearing a gunshot. The knife could also be used for other things,such as setting traps for the soldiers of the opposite side and all sorts of small tasks you might need to perform during lenghtier missions. I also think that the outcome of struggle between the sentry and the soldier executing the takedown (from now on referred to as "attacker" and "defender") could be decided by things such as;
the type of the units the defender and the attacker are (for example it would be more likely for the struggle to end up in the favour of a special forces operator if he surpises a pilot from the behind and vice versa)
the equipment the attacker has (for example it would be more likely for the attacker to succeed if he has a combat knife equipped and if he doesnt have one equipped,then the attacker would have less of a chance to come out of the struggle victorious).