I leveled to 24 as a Seer in Beta. I have to say, it wasn't difficult at all. In heroics and flashpoints you'll definitely need to take on the healing role, but in most normal quests you won't need to heal extremely often, and you'll be able to do a decent amount of damage. Every point you put into the Seer tree makes your Presence stat better, which increases healing and your companion's effectiveness. So if you intend to level as a Seer, you will require a companion with you when playing solo to be effective.
So, I'd actually say 10/10. Unlike a DPS spec, you have the ability to heal your companion and yourself, really increasing your survivability to make up for the extra damage you won't be doing. I might even say 11/10