I've had city guards help PCs who were the victims of crime. One thing I recommend is making them tough enough that it doesn't take 10 to arrest some NPC thug or thief - I use 3rd level warriors w +2 STR & CON bonuses as the typical guard, veterans 4th, elite & officers 5th. That way they can do their job of enforcing order vs minor threats, and be a real help to low level PCs. Low level N/PCs slaughtering droves of city guards is one artifact of 1st ed I'm happy to abandon!