Serious gamers -- I doubt they'd be happy with Android games. Kids -- no killer marketing/mascots. Casual gamers -- likely wouldn't care enough, their phone is good enough.
However, if this gained an install base, I'd assume it would drive the nearly non-existance premium purchase market on android games up.
[1] - http://www.theverge.com/2013/7/31/4573596/nvidia-shield-revi...
I hate playing casual games on my phone. The lack of physical buttons constantly shits me. I've tried all the various clip-on controllers and have used wireless controllers tethered to the phone, it's just not right.
I absolutely love my PS Vita, but wish it had better apps when I'm travelling, so something like the Shield, which has games for when i'm stuck on a plane or waiting for a bus/train, but also has full Android apps for when i'm travelling, is awesome.
The fact it can run Android emulators with physical buttons is also a bonus.
#2 It doesn't have a horrible controller like OUYA, but actually a pretty good one.
#3 It's portable since it has a screen.
As for the game controller+tablet, first you'd need a Tegra 4 tablet to match its performance, which costs $500 or so, which may be worth it since it's a tablet, and you're getting more out of it, but you won't be getting to play Steam games on your TV with it.
So I guess it depends how much of a Steam player you are, too.