Why would they choose to use Power instead of x86_64?
x86_64 is used on PS4, XOne and PC, so it makes easier to port games and use the best the hardware can offer since it's been used for decades in gaming industry.
AMD also seems to have the best offers for consoles performance/price wise, no wonder both PS4 and XOne use Jaguar.
That seems to be a shoot at their feet.
No way Tegra can keep up with PS4 and XOne.
It's probably gonna be too expensive for a handheld and too expensive for a console, making it a failure on both.