As fas as I can see the only real advantage for doing so is lowering your influence upkeep, so no guards or timers are necessary.
For an enemy the colony is a nice prize as before. Actually probably an easier target as defenses usually go down to zero. That makes "hostile" species exchange much cheaper and also generally feels wrong. Also reinvasion should cost something.
So I also did an implementation PR-3336 which keeps the max shields, max defense and max troops values.
I think that solution would also be way better than what we have for conceding. Currently you either concede and give your former enemies a great prize for low cost (also unbalancing the galaxy) or you painstakingly try to bring up the cost. If you could simply set everything to defense and concede and the defenses are still up at least your enemies have to pay a reasonable price in troops.
edit1: typos, emphasis
edit2: PR-3336 got merged