It's like Apple TV 2 vs Apple TV 3. You're hard pressed to notice the difference. Both are the same platform, but the newer one can display 1080p.
That's a sweet spot, people don't know the console but can consider giving it a try because of its price. Choosing Tegra 4 would probably increase the cost of the hardware.
Maybe in the future we will be able to buy a Ouya 2 (backwards compatible) with Tegra 4 at the same price, that would be wonderful.
The Tegra 4 sounds like it might be a platform for hardcore games. Not with every last bit of the latest 3D glitz enabled, but good enough to run a version of most recent games.
From the user's point of view, maybe not. If you do not do anything which requires much CPU power, then the difference may be between waiting 6 milliseconds and waiting 1 millisecond. I believe the parent's question was to know which kind of task requires the CPU power.
Realistically,its probably already a challenge to ship the console with existing hardware, so it makes sense