Those of you who know me know that I am not normally a fan of marinades for beef. For better cuts of meat, the marinade is simply a waste of time and effort. Pepper, salt heavily and place under high heat and you are done. Having said that, several summer favorites, including flank steak, hanger steak and London broil do benefit from being marinated. I had a chance to cook all three (and some chicken, which received the same treatment) during our "Fiesta" over Memorial Day Weekend.
Marinating tends to break done some of the more sinewy meats, such as flank steak, and makes them more tender. For very lean cuts, such as London broil, marinating makes the meat more moist and also makes controlling the doneness easier. I have seen non-marinated London broil go from "blue" to "black" in a minute or two. Although these cuts of meat are less expensive than classic steak cuts such as a porterhouse, they are often more flavorful and need no help with seasoning because they already have a great beef flavor. Because of this, marinating should be an aid to cooking in many instances; not a substitute for proper seasoning or sauce once the meat is cooked.
Beer is my favorite marinade. I usually add a half teaspoon plus of salt to each can of beer (it should taste like salty beer, not salt. Sometimes, I add half a squeezed lemon or lime to each can as well. I try to marinate for at least an hour, but will do it for as little as a half hour or as much as four. I dry the meat off and salt it prior to grilling: this allows it to have a proper char and adds flavor.
Don't get me wrong. There are recipes I love, such as Korean short ribs, that call for a complicated and full flavored marinade. Having said that, a simple marinade can really help a less expensive but flavorful cut of meat shine.