Plausibility depends on storyline universe.
We aren't limited to FreeSpace...space marines could conceivably do severe damage in science fiction (AND in real life). A bazooka-like homing weapon fired by a Marine in space could possibly even take a spacecraft apart in one shot. This is especially true if that universe's spacecraft are small and compact, and/or vulnerable to projectile attack for whatever the author's reason. It's even more possible if the warhead contains a small nuclear device.
Of course, you have to ask yourself what people would be doing fighting in space suits if fighters were present. Desperate times, perhaps. There could be missions featuring Marines planting futuristic C-4, or sabotaging enemy ships with drills, saws, cutting torches...
...which gets me thinking.
Leeches.
Is it possible to create drone-like enemies that latch onto ships and slowly damage them until they are physically removed from the ship? This behavior could be useful for a great number of things.
