Benefits of using MacBook Pro with discrete graphics
You already have the answer to your question.

As far as I see it, the dGPU will be only beneficial for demanding games, heavy 3D work...

If you're planning on using Solidworks, Maya, Lightwave, or any 3d creation or CAD application get the Iris pro one. It is significantly faster for those tasks and not that far behind the GT650M-750M for gaming

The iGPU, will reduces the complexity of your system. Eliminates the graphics mode switching. Less power consumption, prolongs the battery life.

On the math side:
 $2300 vs $2650 is 8.6% or $350.

What can you do with $350?

Buy some software (Games) you always wanted.

Buy USB stick, or External Drive for back ups.

Keep it for rainy days :)

On the other side: 

The $350 is not for the GeForce GT650M alone. Check the system specs in more details to find the differences you care about.

My vote would be for the iGPU since it comes with newer model that brings other engineering improvements with it.