ok I am guity of using this exploit.
When invading a planet I always send in a suicide 1st wave of 500-1k troops and have them do something really devastign like mass drivers. They'll loose but they'll wipe out like 4k-6k enemy troops. The catch is since they lost, the planet doesnt loose quality. I then send in the much larger main force in with traditional warfare and win the planet intact. Honestly though, invasions are so hard unless you have vastly superior tech that if this exploit gets taken out, I'd like to suggest that the entire population of the planet should NOT all fight. Maybe it should be a ratio based on militarism/soldiering/loyalty or something.