LogFAQs > #932412702

On One Down the ZEAL friends need a minute or two to melt your defence boosted converts and if you're playing the objective that's really valuable time. On OVK/Mayhem they can last ages.
They're a great part of any "No don't shoot me kill my teammates instead" kit with flashbangs, dodge skills, dodge perk deck and whatever else.

Having regular people as hostages allows you to go over the 8 hostage limit since it comes down to how many there are and how many cable ties there are which at least serves to divide the enemy attention again since some of them will spend time trying to rescue them instead of shoot you. I think it also gives you a slightly longer break in between waves.
There's also this priority list for trading hostages. Regular civs are the top, hostage cops in the middle and converted cops are last, it's always nice to have one ready so the game doesn't make you trade away a convert and force you to get another.

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Still waiting on that third threat