Usually when I marinate any kind of meat (usually chicken or steak), I always just stick the whole piece of meat in marinade and let it sit for X hours. My question is, is it possible to achieve better (more flavorful, or juicy) results from marinading if you poke holes in the meat before marinating to allow the juices to flow INTO the meat? What are the benefits and drawbacks of said approach?  For the purpose of simplicity, lets keep the discussion to cuts of chicken and steak, cooked on a gas grill. 
Most flavor molecules from marinades don't penetrate much below the surface of the meat.  (Salt and to a lesser extent sugar are exceptions: they will gradually work their way in deeper in long marinades.)  Anyhow, if you actually want your marinade flavors to penetrate more than about 1/8" into the meat, the only real option is injection.  But if you don't want to do that, poking holes or even gashing the surface with rough knife cuts will help a bit.  You can see some photos at this link, showing how a colored dye (whose molecule size is as big as typical flavor molecules) doesn't get below the surface, but gashes and holes can help carry it deeper.  I don't know that there are any real drawbacks to the method, other than having a craggy surface on the meat.  As Jolenealaska pointed out in comments, the idea that significant moisture loss occurs from piercing is mostly a myth.  (For more detail on that, see Myth 6A here.) 