Silenced weapons (crossbows, silenced pistols and whatnot) never really seemed to do anything different than any other. You fire, and the enemies notice you (because obviously you shot/killed one of them). I presume that it makes the difference in that, while it is still your turn, you can make continuous stealth attacks, while with an unsilenced weapon, your stealth bonuses go away after your first shot. I haven't managed to find it anywhere explained, and I haven't properly tested this yet, so here I ask.