I don't know how useful spikes would be, but they look menacing and designs involving them are fusion-powered so they must be good for something, I guess. Tapering radiators (and propellant tanks) is about keeping everything inside the shadow of the radiation shielding so you don't have irrdiated coolant circulating near the crew.
Should be a couple of example pictures from projectrho below.
For what it's worth, I think I know why the Attack Vector ships have spike-shaped radiators. (besides looking cool)
The engine is some sort of fusion torch. You've got lasers or whatever beams to ignite the fusion (inertial confinement), mounted on the skeletal framework of a magnetic nozzle which contains and directs the exhaust. That engine must survive hellish radiation and heating, which is why it's mostly open to space. However, some physical elements, like the magnetic nozzle elements and the beam guns obviously still have to be there, and must be heavily shielded. If you look closely, those spikes are mounted behind the beam guns. That way the radiators are fully behind them and benefit from their shielding.
From I remember about that game as well, the spike radiators were used in conjunction with heat sinks. When your spacecraft ran out of heat sink capacity and you extended your traditional radiators, it was a sign of surrender/defeat.