He'd be willing to let everything his colony had established be destroyed along with all its people to encourage some other colonists he knew nothing about?
What was the point of getting killed at the Alamo? Nobody survived, but the senseless defense sent a message.
That's among the most common reasons to put up a defensive fight: to be remembered. Generally, you don't get attacked unless the attacker is certain of his victory, after all, so the defense achieves nothing in pure tactical terms.
The difference there is, the Alamo was a battle between opposing human forces. Sure, it may not have been exactly even, manpower-wise, but still a mere matter of one group of humans against another.
In this episode, however? It would not have been a battle, but rather a slaughter. It would have been literally impossible for the colonists to beat the Sheliak, because as Data pointed out, the Sheliak could have simply obliterated the colony from orbit. A slaughter like that would be completely pointless when it's so easily avoidable. The human colonists would not be "standing up" to the Sheliak by fighting them; they'd be mindless sheep agreeing to be wiped out. Who would want to be remembered like that?