The biggest reason that its basically all Snapdragon at this point is because Qualcomm has an integrated cellular radio in the SOC, which gives you a lot better power optimization. NVIDIA only recently got it integrated (on a throttled back Tegra 4) and Intel is about a year away. There could definitely be benefits to an x86 processor (once they get the graphics on par - maybe they can license the Nvidias mobile Kepler architecture...), but at present the power drain is too high
Edit: Just to add about the powerfulness of Intel vs Arm, it isn't even close. Intel blows away arm on a per core basis, but the power consumption is higher. Overall, though, the new atoms can stand on their own vs the other chips, they just don't yet have the radio integrated so the stand alone radio uses a good deal of power