Starhawk writes: "you are right about the fact that it's probobly so far in the future none of us will ever be right about what will be there."
Actually it COULD be happening right NOW just a few light years away.
Seriously, though, given the uncertainties of future technology it's probably better to confine ourselves to the fictional Space Empires game "universe"; those of us who enjoy using troops can try to rationalize the use of ground forces (actually air/land/sea forces) in that limited context.
Off the top of my head I can think of two possibilities to start:
1) Suppose most of the planetary population would rather be "red than dead", but government and/or military officials insist on "death before dishonor" (as, for example, the Nazis and Japan's military at the end of the Second World War). Suppose further that it's impossible to take out all the die-hards from orbit without taking most of the planet along with them, but ground troops can accomplish the mission.
2) Suppose that, for whatever reason, domestic politics requires a sincere attempt to minimize "collateral" casualties, as for example with the US in Iraq, 2005 (as opposed to Japan, 1945). If SE IV sensor/weapons technology makes space-based pacification excessively bloody, then politics, rather than military considerations, may dictate the use of ground troops instead. Obviously this isn't in the game itself, but players can role-play, and fiction based on game events can include it.
Game-based fiction can also include interstellar politics, e.g. neutral empires that would likely join the enemy out of fear if the player glasses too many worlds. A writer may also use a low-casualty policy to explain the relative ease of winning the loyalty of captured populations in the game: suppose the Terrans, for example, capture Pyrochette worlds with troops, but the Pyrochette AI nukes its own (former) people just to nail a few occupying Terrans. That might not convince every Pyrochette to love the Terrans, but it might produce enough Quislings to make planetary occupations a lot easier.