Well, from my observations so far, this setting is an improvement, as it stops the CPU and mainboard / RAM from overheating, which they would regularly be sitting at the very brink of before. The MX 250 does seem to handle the load of 180,930,194,964,480 % 3D usage (before, it were about 120% at best) quite well, as the temperature stayed about the same as before. I guess the usage of the integrated GPU stems from being the connection to my monitor via the HDMI port. But the temperature tells me that the nominal increase didn't do much to the temperature load of the iGPU, perhaps even lower it a few degrees.
The overall performance didn't change much, aside from reducing the amount of thermal throtteling to a minimum. Before, I had to wait for a few minutes after the graphis had loaded in to get it all to settle down, as any action would result in a series of screen freezes. Now, this is reduced to some slight stutter. Still not optimal, but about a minute of waiting there can alleviate that easily.
This all hints that
@0threeleven had the right idea. If left unchecked, the discrete GPU and its specialized RAM would be mostly ignored and trundle along doing some minor computing tasks, while the integrated GPU would do all the work, stressing the CPU, the RAM and the mainboard in an unduly manner.