nah. saturn showed surprise launches kill.
i think it also creates the situation of, lets say Xbone launches in August (anyways rumors are they have problems, so unlikely). Then you just have people perceiving PS4 as the "true" next gen and waiting for it, or vice versa if ps4 went first. It's a subtle but real thing.
Of course if you have the most power it doesn't matter lol. If Xbone was 3.0 teraflops none of that would matter. But it's 1.2.
I think the Halo franchise killed some of their hype when they moved launches to Sep from Nov for example. I was glad to see Halo 4 going back to Nov.
You dont really have time to build proper hype in August, it's hot, people are outside, there's a reason big games dont launch in summer, etc etc. The holiday season is natural hype time.
I think all of this is fairly minor. In the end it's the product that matters not a launch date. But, it's still negative to launch too early imo.