Soraka is perfectly balanced. There's more to it than heal: right positioning, hitting Qs and map awareness for ult. The heal amount has been recently nerfed anyway, and if you are saying to increase the CD and the heal at the same time, that's not a nerf at all, because the less W is used the better (as it cost 10% of her max. health).
If you reduse what makes her a "heal bot", then what is she ? Most supports which have healing spells, have other kind of spells as well: Sona has damage and CC, Taric and Alistar heavy CC etc.. However, I'm talking about the main kit, so even thought Soraka has a slow and a silence, she can't rely on these as a basement kit of both laning phase and late/teamfight, as mostly her Q is only to heal herself prior to using W.
In high-elo (Platinum+), most supports laning against Soraka take Ignite along with Flash as Summoner Spells, the same way some ADCs build Executioner's Calling early in the game. You probably don't know how frustrating it is to know that your heals are reduced by 50% almost every fight, but think about this: an item similar to Executioner's but which reduces all your damage by 50%, instead of your heals.
As an off-topic, if you were/are a World of Warcraft player you'd know that healers are the main focus in battles. Of course, WoW is not LoL, but it's just a good reference to how important healers are in a game.