Stock type disc/drum combination valves are designed to mate the different physical characteristics of the stock disc and drum style brakes.

Discs always have their pads touching the rotor so it's virtually instant on. Drums do not have the shoes touching the drums so there is some delay before application. One of the functions of the stock combination valve is to "HOLDOFF" the pressure to the front brakes until the rear shoes touch the drums. This allows simultaneous application of the brakes at all four corners. Using this function in an all disc setup will apply the rear brakes before the front brakes. Not good.

Stock modern style drum brakes are self energizing, which boils down to this, they do not apply braking force in a linear fashion. The stock combination valve will slow the rate of increase once rear line pressure hits a "HINGE POINT". Using this function in an all disc setup will make the rear disc non-linear in relation to the front brakes. This may or may not be an issue as weight tends to shift forward decreasing traction in the rear, only testing can determine if this is so for your setup.

Those are the two major issues using a disc/drum combination valve in a disc/disc setup. I would not do it, but I am sure there are plenty of people who have and will tell you "it works fine", those people don't understand how the stock system works and cannot comment knowledgeably on it. You now can, because you know and you can now make your choice.