[dmm]

I might be mistaken, but:
1) If your enemy has weapons with a range greater than 1, the AI will do the intelligent thing and attack your vulnerable troops with weapons rather than your harmless shielded troops; and
2) If you don't have many troops, then the "front lines" might contain some weaponed troops, which the AI will kill first, and then continue killing as the weaponed reserves patch the hole.

Regarding shields protecting a planet from ground assault:
I don't know why, but planet shields NEVER work that way in sci fi movies. It's always:
"Rats! Their shields are up. Now we can't bLast them into oblivion with our 1000-dreadnought fleet. Prepare for ground assault!"
It is never explained why the troop ships can get past the planet shield to drop their cargo, but the dreadnoughts can't go past and fire from just inside. I think it has something to do with Newton's Law of Good Theatre.