Problem with kill swapping is that, especially early on, it isn't actually as valuable as it feels. If the guy you would swap with is running all the way back to base, you have two avenues:
1) Stay in lane, and get more gold and more exp due to not having a contesting DG (No need to fight, can always attack creeps, etc)
2) Try to kill the DG and Maybe get a bit of Gold. The 250 experience you get isn't much compared to the xp of staying in the lane though.
The first option contains little to no risk. You cap any flags there, you get gold and xp, and can potentially fight off any new DG that comes.
Option 2 however is extremely risky. If th enemy has a Pot and are simply being sneaky, or if you miscalculate, or any of the above, you end up losing all of option 1's benefits, and instead GIVING them to the enemy.