Bob's discussion with Harry about the nature of Soulfire and it's applications to magic in general might be relevant here. As Bob put it, adding Soulfire to Harry's usual magic was like adding rebar to concrete. The combined effect was stronger and more effective than either one was seperately. Perhaps Summer's power is like that. Good by itself and exponentially more powerful when in combination with other power.

Remember what Eldest Gruff did to one of the Fallen? A flash of golden summer light mixed with foest green and BOOM. No more Magog.