It's just fine for almost all users and if you want a discrete card you can get one.
Also, i'd say if you think about your usage more seriously, if you're considering a DGPU in the machine, what you probably really want (of course there will be exclusions to this - and this is assuming money isn't an issue) is a sonnet thunderbolt enclosure and a desktop card in that. I'm just waiting for apple to put a PCIe slot in the thunderbolt display (will likely not happen though :-\).
DGPU on battery doing any sort of work that couldn't be handled by the the iGPU will burn through your battery in an hour or so.
So the case for dGPU is really those who carry their machine to somewhere away from their home/work and run heavy 3d while there.
I'd say most who use dGPU are doing so at a desk, on AC power, where they could have a thunderbolt connected GPU.
A friend has an 2011 MBA11 with sonnet enclosure that runs triple-head gaming just fine. Youtube for something like MBA triple-head, he runs a demo of Dishonored.
Of course, YMMV, but that's the conclusion i've come to after having a half-decent dGPU in my portable machine for a couple of years. The internal mobile dGPUs are always sub-par and just burn a heap of battery - so don't bother and go external desktop GPU...