Beam turrets use the simultaneous shot text because a player requested that they do so
In the initial implementation it didn't actually say anything about the multiple beams, iirc.
Anyway, they don't actually use the same NumberOfSecondaryShots logic as other stuff (like MLRS's or Light Starships) at all. It's a totally different mechanic whereby the central beam runs along a line segment from the firing ship through the target ship out to the beam's maximum range, and the "simultaneous shot" beam end points are computed as points (roughly 40 world units apart which is somewhat smaller than the diameter of a fighter, iirc) along the perpendicular line centering on the central end point.
So the idea is that they work best against fairly tightly packed groups of ships, or large targets with lots of health (like transports). If firing against a lone core bomber you'll be somewhat disappointed (though the bomber will still feel very unwelcome). This potential inefficiency is part of what balances their rather ridiculous overall attack power. A Mk IV does 40 * 50,000 = 2,000,000 damage per shot; if that were always applied in the traditional secondary-shot fashion it would almost certainly have to be nerfed.
