Honestly, I don't buy the idea that the Summer Court would be opposed to genocide.
They're about growth and progress sure, but they don't seem to hold reverence for any particular species. To quote Bob in Summer Knight
"A war [between the Courts] could start the next ice age, or set off an era of rampant growth."
"That last one doesn't sound so bad."
"No. Not if you're an Ebola virus. You'll have lots of friends."
The idea that every species matters is a distinctly human one, and I doubt the Summer Sidhe care. They're not conservationists. In fact, if anything, that would fall under the kinder aspects of Winter - Stasis.
If a species, such as the Black Court, is no longer suited to it's environment, it's natural that it will be supplanted by more powerful, better suited creatures. The Black Court is an evolutionary dead end, and the sooner they're cleared away, the sooner new life can replace them.
And that's even if you accept "undead" as a form of life, which I don't think is necessarily justified in the Dresden Files.