I'm playing Payday for steam's free weekend, and notice that I can take civilians hostage. Other than preventing them from getting up and getting shot, is there any point to doing so?
The hostages system gives you the opportunity to free one of your teamates from jail. Since you can't really "die" in Payday, this is their "revival" system.
You can take civilians as hostages by shouting at them and zip tying them, or even cops. Taking cops as hostages is a bit more tricky; you may need to hit them first (with a non-lethal shoot or with your melee attack) and then shout at them until they handcuff themselves, which will save you a pair of zip ties. Also note that you cannot accomplish that during an assault (indicated by the red triangle at the bottom of the screen).
If one of your teammates gets arrested, you'll have to wait to the end of the assault plus the eventual time malus (if they killed civilians) to proceed to the trade.
Be careful with your hostages, as they are slow to be moved. So if you leave them without surveillance, they can be freed by the cops.
You should always get hostages (police or civilian) and ensure they are nit only intimidated, but tied down using cable ties for civilians or the cop's ties for a dominated police officer. Once you have got a hostage, make sure you guard them, or the police will attempt to free them. If you kill a hostage you get fined, and you also get a trade delay which means it will take Bain longer to negotiate with the cops to trade you; if you end up in custody. If you kill no civilians, the trade delay will be 5 seconds, once the assault wave is over. Once Bain has negotiated a hostage trade, a remaining team member will need to go up to the marked hostage and free them and the teammate in custody will be released, with full health and ammo, but their equipment will not be replenished.