Let's put it this way: if you cook the meat in its marinade, you'll steam it, not roast it. Not very pretty. Even if you drain the meat, dry it, brown it, and then add the marinade back, as Phil says you'll be braising it, which is indeed not the best way to treat tenderloin.
What's in your marinade? That can make a difference as to whether -- or how -- you can use it once the meat is out.
Best thing to do is drain off the marinade and dry the meat (paper towels). Then you can quickly brown the meat on top of the stove before sticking it in the oven to finish cooking. Since pork tenderloin cooks so quickly, you might not have enough time to baste it: iirc, raw marinade used to baste has to cook for at least 15 minutes. If you boil the marinade first, it will be safer, but still might not really add much to the meat.
If you strain the raw marinade and use it as part of a pan sauce (deglaze the pan after you remove the meat and set it aside to rest), you'll still have to cook it long enough to take care of any bad stuff. But then you may have a problem with the proteins in the marinade (in the form of meat juices that leached out of the tenderloin) clumping up and turning ugly-looking. So you might just as well discard the marinade after all.
"Notorious stickler" -- The New York Times, January 4, 2004