It's kind of like the flak frigates. They have very high listed DPS on the infocard, but it assumes that all 4 sides of the ship are able to fire at something, and even if they do, it'll be distributed over a number of ships, rather than just one.
The listed DPS is correct, but doesn't mean that that's how much damage one target ship will take... it just refers to the overall damage output of the frigate.
I wonder, do the side beams focus-fire as well? If you have a group of 10 illuminators in a row, primarily targeting some ship right in front of them, then if there are multiple ships to the sides, do all the side beams focus on one of those ships on the side? Or do they just fire at some random target in range, after taking into account armor type?
If the side beams don't focus-fire, they're virtually worthless.