Most of all I believe Moridin wanted to gain Rand's trust by revealing this secret.

It seems possible that Mordin even thought that Rand already learned about this final balefire-death, e.g. from Asmo, LTT or Semirhage. After all, Rand also used balefire against Rahvin.

Besides that, Moridin might consider that this revelation won't matter much anyway, because any Forsaken who will die now won't be transmigrated anymore, since TG is simply too close (Osan'gar apparently wasn't transmigrated).

Finally, it might of course destabilize the Pattern, if Rand and other Lightside channelers use balefire more deliberately, which helps the Shadow.