Every modern game has film grain as a check box option in the options menu. It should absolutely be an end-user control.
The more control you take away from the end user, the less enjoyable it is.
Yes, I understand from the perspective of the designer that it should utilized as and artistic enhancement for the mission. But it should not be something available ONLY to the designer. What if the player wants to have it turned on in other moments throughout the game?
The checkbox (in our case, a commandline flag), should function as an additional option to afford the player more control over their playing experience.
I would merely suggest that the commandline be included as something that works in additon to the spex.
The fact that all of these neat effects are available only to the developer, and not the end user is, in my not humble opinion, selfish.