Just being a moaner
You see, jumping in and out is different from withholding.
If you jump in and get straight out of a pool, you aren't withholding, although you aren't in the pool anymore.
With Swooping Hawks and the Rubber Hawking technique, you do it like this:
1. Roll for reserves. If they are available, proceed to step 2. If they are not, proceed to step 6
2. Commit to a point where the Swooping Hawks will attempt to land.
3. Drop the grenade pack, resolve hits and wounds.
4. Roll scatter for the landing of the Swooping hawks.
5. If any swooping hawks remain, Skyleap.
6. Await till next turn and return to step 1.
As you can see, you still have to place the swooping hawks. They aren't invincible - if you spread out your forces enough, they might land on a squad, or impassable terrain, or one of their own squads, and lose models or even the entire squad.
If you do the Rubber Hawk technique, sure, they might not be able to shoot you. But you might land on top of something that will destroy. Worth the risk?