Only a subset of encounter cards remain in play. Many have a revelation effect and are immediately put in the discard pile. For this reason, Darrell can get a lot more use out of his camera in higher player count games where more encounter cards are seen each turn.
The fact that he can place the evidence on enemies and treachery cards that investigators drew at any location (That hopefully have clues on them!) makes multiplayer the place that this card shines.
Finally, don't forget that many weaknesses are treachery cards so you can benefit off of your allies' misfortune. ;-)