According to the "Common misconceptions" section of the "Tips, strategies and other information for PD:TH" guide by DorentuZ` and Tromboncino:
Destroying cameras
Destroying cameras does not make the cops less aggressive nor does it reduce the number of cops on the map or make them path towards your position faster. The effects differ per map (see below), but in general they either stop or delay the spawn of certain scripted special units.
From the "First World Bank" section of the same guide:
Cameras
There are three distinct "sets" of cameras in this map and each set stops a different event. The first twelve cameras, the ones you see before you have entered the vault area, control the event that spawns snipers after you've emptied the vault. Destroying these cameras reduces the chance of them spawning to 45% on Overkill and above and nullifies the chances on the difficulties below.
The second set are the cameras also used for overdrill (and the one in the hallway before it). Destroying these stops two elevator doors from opening and hence stops spawns from there.
For the final set of cameras I quote Frankelstner:
Breaking the remaining vault area cameras disables the spawns associated with the first elevator on the right and the second elevator on the left. Breaking the cameras before the doors have opened has no effect. Because of that, depending on your luck with the initial 5 cameras, you might disable all elevator spawns while at other times breaking the final cameras has no effect whatsoever.