I don't see how everyone suggesting who they'd cop is any different than people suggesting who they'd doc/cop/kill as seen in other games. As far as I'm concerned, it's a simplified, flavor-filled read list. That said, I do agree that generating leads by comparing who died to everyone's scum choice isn't entirely useful.
Basically the argument is that introducing FBI Agent targets could result in having a harder time analyzing the night kill selection as a result of the WIFOM that comes into play.
In my opinion though, I don't think it will affect NK selection much, and Town stands to gain more than it potentially loses; the SK knows that the chance of hitting the target they want to hit can't really be determined through FBI targets alone, and them taking the chance to attack someone that has selected them as a NK target could result in their demise if they try to play with WIFOM.
On the flip side of things, Town knows that they can't establish what the kill was based on FBI agent selection. Likely, the Town will analyze game play, opinions, and why people are coming to the decisions that they're making, in order to determine the appropriate lynch. The FBI agent selection I feel adds an additional layer of discussion that could be beneficial for scum hunting.
I don't mind doing a round of Cop/Doc/Kill instead though if that's what people prefer, lol. Basically the same thing without the added bonus of being able to follow the trail of the FBI Agent if he dies.