There are other reasons to to give a meat marinade a thorough boil. As the marinade seeps into the chicken (in this case) the chicken blood and other juices will flow into the marinade. When it heats you will get a "scum", just as you do when making a stock. Boiling the marinade (and I don't mean a violent, rolling boil--just enough to agitate the surface a little) will bring that scum to the surface and allow you to remove it. Better to do that at the front end than have that "albumany" stuff floating around in your gravy boat. This process will also act as a "mini-clarification" and help clean your sauce as well.
All of that said I question why you would want to turn the marinade (as you describe it, it is also a cooking liquid) into a sauce. Obviously there is the idea flavour building, that the liquid should have a alot of flavour. My worry is that it might taste on the burnt side. Also, you didn't give the proportions of the ingredients. You might very well have nothing but an oilly mess that wouldn't make for a nice sauce at all.
If you want to make a tastey sauce based on this method I would try removing the chicken from the pan, setting the pan on a moderately hot element long enough or the solids to cook onto the bottom of the pan. The fat will (should) separate. Pour the fat out of the pan and then deglaze the pan with wine, stock or a combination there off. Scrape those tastey, brown bits off the bottom and let them dissolve into the liquid and cook it down a little. You get bonus points for straining this liquid off before serving by the way. If you like, finish your sauce with some of the same mustard and herbs that went in the marinade.