Please humor me with some viable reasons
1. Its a game for Pete's sake. Before you call "God why are the Enclave so Dumb?!" Consider that the area varies according to what the current conditions are. I've wiped the area out before and then triggered the Enclave drop. Which would make sense ("Alright our chief enemy just got done fighting a bunch of mutants, he's weak and low on ammo. GO GO GO!"
Simply because the person in the video triggered the Enclave drop sooner and made it a three-way fight (well two way, since the player doesn't really get involved) in which the Enclave sends its troops into a death-trap, doesn't mean that this always happens.
2. Once again depending on the character level, there can be so damn tough Enclave soldiers which spawn from there. Hellfire etc.
3. Its a scripted event. Blame Bethesda for writing the Enclave to make silly decisions like this. You can't blame it all on tactical ineptitude. Bethesda makes a point of trying to show how the Enclave is the most "powerful" enemy in the capital wasteland, but at the same time, writes the Enclave so they do things like drop their soldiers into hot-zones.