[Noble713]

Abandoning planets can be useful for raiding enemy systems. You come in, capture the planet, load any population you want, abandon it (scrap the facilites if you don't expect to secure the planet for a long time), and go home. If the enemy takes the planet back, he now has to ship in people to run it, and may or may not have to totally rebuild the facilities.

Also, if you play as a species that wants to exterminate all sentient races (like I do sometimes), it's a great way to wipe them out without needing neutron bombs. Actually, I wish troops could kill off a planet's population like in MOO2. I LOVED that feature.