Dual use Ion/electric drive/weapon?

Ah, I see part of the confusion. The plan isn't to use these against enemy ships: the question is their utility against land forces, like infantry and tanks. These ships land and then can move around on their own power (they're like giant snakes/worms kinda thing, with the space rockets in their "head") I'm wondering what happens to ground targets if the ship points its head at them and fires in an earth-like atmosphere.

This would probably be helped if I had some example of the kind of engines and power being discussed, so I go into the editor and see what I can whip up.

Exhaust gas disperses too easily, even high pressure, high temperature, high velocity gas. The sound vibrations from your roaring engines is a lot more threatening (think the distance a Saturn V's engines can shatter glass). Still, this ranges is pitiful against armoured vehicles, especially ones anticipating this.

Furthermore, spacecraft are powerful, yet fragile things. Blasting foes from orbit before deploying troop transport shuttles/pods while the main ships provide fire support makes more sense. That capital ships can land on and take off from planets doesn't mean it makes sense to do so. Putting relatively expendable shuttles in danger is better than risking the capital ships that have to ferry troops and material between planets, provide orbital fire support (which they can't do from the surface) and cost a lot more than a few shuttles.

KInetic strikes have the advantage of being more targeted and producing no fallout, though - which is useful if you want to avoid collateral damage. Also, I think with the same total energy at impact/detonation, a kinetic strike is more penetrating than a ground-detonated nuke - due to having quite a lot of downwards momentum. For example, a 1 m diameter tungsten sphere (mass ~10 tonnes) entering Earth's atmosphere vertically at 30 km/s impacts at 16 km/s, delivering 304 tonnes TNT equivalent and making a crater 118 meters wide and 25.2 meters deep in crystalline rock. Air blast, thermal and seismic damage is negligible at 500 m distance - only windows will shatter.

To compare, a 304 tonne nuke surface burst will make, according to Nukemap, a negligible crater. The airblast will shatter windows out to 660 meters, the thermal flash will give people burns out to 820 m, and people will receive a more than 50% dose of radiation within 680 meters.

So, kinetics will be better than nuclear strikes, I think, against small, hard targets. Not to mention that a few tonnes of tungsten rod plus the energy to accelerate it is probably cheaper than a nuke, even a small one (IRL at least).

Most of our spacecraft have a high-thrust burn time under 15 minutes. You'd never use the main drive torch as a weapon, even if (as others have stated) it would be practical.

Space-warfare Capital ships getting into a planetary atmosphere is unfeasible. You'd clear the landing zone and suppress orbital/ground-to-space defenses, then use SSTO airbreathing liquid air-condensing nuclear spaceplanes; and off the top of my head you'd only deploy these on Earth or Venus. 

One thing to note is that space-combat optimized ultra-high velocity sandblasters and super high frequency lasers are of dubious use in Orbit-to-Ground strikes; a 100km/s 1 grain VanChrome pellet will vaporize in any thick atmosphere far before hitting the target. Likewise, thick atmospheres are not at all transparent to our frequency quadrupled UV superlasers.

Hyperoptimized micromissiles with millimetre thick skins will not survive re-entry, though heavy capital ship missiles with thick composite armor will easily double as Orbit-to-Ground munitions.

You'd probably clear the orbital space using Space-to-Space warships, with a dedicated Invasion/Siege ship equipped with anti-planet weapons (kinetics firing slow, sub-10/20 km/s heavy shells that can survive reentry, long wavelength lasers) and invasion troops. The only way an invasion could be plausible between near-equal entities would be the liberal usage of WMDs; otherwise the defender's numerical advantage is too ridiculously high.

If you add an induction coilgun to the end of of a plasma drive, might you not be able to push the exhaust into a focused beam?

www.youtube.com/watch?v=AVUmF2oBYWo

For ground bombardment, nukes and KEPs are the way to go. Lasers lose to much of their already terrible energy efficiency. A mix of nukes and KEPs allows for both efficiency and tactical versatility. KEPs and nukes allow both precision strikes with concentrated fire and wide scale destruction with distributed fire. They are also significantly harder to counter than lasers.