Let's put aside pickling, brining and making seviche, techniques that warrant their own columns.
Normal marinades do not significantly penetrate the meat, no matter how long they are left in contact.
Food scientists such as Harold McGee and Hervé This have measured the penetration and found it to be staggeringly small.
An experiment conducted by This at a meeting of European chemical societies last year showed that after eight days, a slightly salty marinade gave meat a somewhat mushy surface but penetrated less than 1/8 inch into the flesh; a mildly acidic one (such as a wine- or vinegar-based marinade) showed next to no effect.
The lack of penetration by the marinade refutes the idea that flavor enters along with it. And doesn't it also mean that the tenderizing effect is minimal?