You shouldnt really have mana problems with Soraka, unless you spam Q for some reason. If you use Exhaust on their AD you will win a trade you wouldn´t have won otherwise, you can use it to let your AD escape. If you exhaust their AD in a teamfight in endgame, they won´t do any damage at all for long enough to kill them.
Ignite is often better than heal on ADs but it´s situational.