If you boil it right after using it as a marinade it should be fine safety-wise, but not all marinades will make good sauces flavor-wise. Also, sometimes enough proteins will be left behind from the meat that, once cooked (and therefore solidified), there may be undesirable shmutz in the sauce.

I think it's fine if you cook it. I use marinades to make sauces all the time.

When I make a marinade of ginger, soy sauce, garlic and brown mustard for my salmon, can I use that marinade for anything else?

The directions say to discard the marinade. But, if I've been marinating them in the fridge for 2 hours, can I use that sauce to flavor another dish like rice or something?

I always wonder this, but never do. Chicken would be kind of gross to use, but even then, if it's cooked, I don't see the harm. What do you think?