Why where the Teleri Ships Irreplaceable?

[Maédhros]

He dared to swear an oath that killed all but one of his sons and drove the last utterly mad. He dared to slaughter his kin--and if they were wrong for refusing him the boats, he was a thousand times more wrong for raising sword against them.

What mattered is that he dared to do what he thought was the course of action to meet his end. His purpouse was such that anything came in second to his goal.
The deeds were bad, the concept of following your goals is not.