In the basic Pathfinder magic system, there are two ways to guage the power of a spell; the spell's level, and the caster level of the spell-user. In Spheres of Power, we also have two ways of guaging power level; caster level, and the number of talents applied to the effect. Usually, there are only so many talents that can be applied to a single effect; with the Destruction sphere, after you've gained about 3 talents, taking more talents in that sphere increases versatility but not necessarily power. There are, however, two exceptions to this rule: conjuring and healing
In Pathfinder core, healing is a specialized job that requires serious investment. I've seen Alchemists and Bards who had the 'primary healing' job, and had to spend so much of their time healing they could use their spells for virtually nothing else. The Cleric of course has the most powerful healing abilities, but pays for that by having a much more limited capacity for blasting, controlling, and creating other effects. The wizard, as the great counter-example, has all sorts of potential spells and effects, but in exchange has virtually no healing powers. The balance concern is that if a single caster could both heal as well as a Cleric and do battle as well as a Wizard, they'd simply be too powerful of a caster and would lessen the chances his teammates have to shine. Since the Spheres of Power system currently has no restrictions on which class can take which sphere, the question becomes how to balance these sets of powers, and make sure the game doesn't become unhinged when any caster can become a healer if they should so choose.
Currently, our plan is to make healing a greater talent sink than the other spheres- while a higher caster level helps, the real way to increase the healing you can provide is by spending more talents on the sphere, gaining more or less an extra d8 of healing per talent spent. Thus, if a caster only wants to sink a couple talents into healing, his healing won't be nearly as effective, especially at higher levels. On the other hand, a mid or low caster willing to spend the talents can indeed become the party's main healer, doing things only the Cleric could do in the base system.
The other great talent sink system is conjuring. I've known a few GMs for whom conjuring was a bit of a sore spot; they'd had so many newer players grind the game to a halt as they looked up which monster to summon and figured out what they could do, and too many experienced players who'd used the spell-like abilities of summoned monsters to double their spell output and destroy all sense of game balance and team tactics.
Thus, we've also given conjuring a talent-sink system; full-casters will be able to summon more powerful creatures of course, but if a caster is only willing to spend a talent or two on the conjuration sphere, he'll have little more than a single weak ally, good for soaking up hits or setting up flanks, but little else. A dedicated summoner, however, can become a walking army, sinking their talents into summoning more creatures or giving a summoned ally even more power. It isn't inconceivable that a caster could sink each and every one of his talents into the conjuration sphere, but doing so will require that player to keep their bookkeeping under control and keep the game from stopping while rules are checked and stats are calculated.
By making these spheres talent-sinks, we've severely limited the amount of power a caster can wield by spending a few talents on the spheres. On the other hand, a player can still turn these spheres into the focuses of his build, he just has to be willing to spend the talents to become good at it. Is it the best way to handle these situations? I don't know; only playtesting and trying a few alternate methods can fully reveal that. But they're systems we're currently looking in to to see how they work alongside the other spheres. As always, feedback is appreciated as we see how these concepts work in action.