I have the late 2013 rMBP 15" with discrete 750M. Whenever I want to use the HDMI out port, I need to turn on the discrete GPU from gfxCardStatus.
This is the one downside to the dGPU models. On the dGPU models, ALL external video ports (Thunderbolt/MiniDP and HDMI) are physically connected to the dGPU *only*. If you have a dGPU and you plug in an external display, the dGPU will be forced on. gfxCardStatus can't do anything about this as it is a physical/hardware engineering issue. On iGPU-only models, of course, the iGPU is driving all external displays.
Another similar issue exists with Boot Camp on dGPU. If you boot a non-OS X operating system on a MBP with discrete and integrated GPUs, the MBP's EFI will only expose the dGPU to the operating system. So when you are running Windows, for example, on a dGPU MBP, the dGPU is the only graphics chip you can use in Windows. Windows won't even see the iGPU. Again, obviously, on an iGPU-only model, Windows uses the one-and-only GPU that is present, the iGPU.
Other than those two circumstances, on a dGPU model, gfxCardStatus allows you to override the GPU setting rather than leave selection up to OS X, so assuming that you 1) never hook up an external display and 2) never use Boot Camp/Windows, you can, in theory, get the same kind of battery life out of a dGPU model by simply turning the dGPU off completely, which still gives you the freedom and ability to engage the dGPU when you absolutely need it. Odds are good that if you are in a position to plug an external monitor in that you are also in a position to supply AC power to the laptop, so I imagine that for most people, the external display restriction is generally not a big deal. The Boot Camp issue, though, seems to me as though it is more likely to be a problem.
-- Nathan