Put simply, real vampires don't sparkle and real orcs don't have pig heads. Over the years, there have been many attempts to defend the rather unfortunate decision to inflict this ignominy on one of gaming's great antagonist races but none, at least to my mind, have been convincing.
The argument that orcs are a pre-Tolkien monster is both wrong and irrelevant. Some confusion about their pre-Tolkien existance exists because of line 112 in Beowulf which reads "eotenas ond ylfe ond orcneas." However, orcneas is translated as "evil phantoms" (Seamus Heaney), "evil spirits" (George Jack and John R. Clark"haunting shapes" (J.R.R. Tolkien - line 90), and "spirits" (Burton Raffel). Others go off half-cocked and site Ariosto's Orlando Furioso as a sixteenth century source for orcs, ignoring the fact that it refers to "a gigantic sea monster called the orc," a singular creature that in no way resembles orcs. Only the name is the same in much the same same way that the clubs used in baseball and cricket share a name with a flying mammal. However, regardless of their origin, it is certain that there were no pig-headed monsters bearing the name "orc" prior to D&D.
The second argument is better but still not especially convincing. This is the argument that fantasy and mythological creatures should constantly be reinterpreted and reimagined for each generation. This is a subjective opinion and therefore neither right nor wrong. However, most people who subscribe to this viewpoint would agree that the should be more interesting, cooler, or more relevant. Pig-headed orcs are none of these. They are allegedly, depending on which story you believe, either inspired by the fact that orc sounds like pork or by an overly literal interpretation of the term "pig-headed." Of course it is just as likely that it was an attempt to make them slightly less obviously Tolkien's orcs in response to possible legal action from their excessive use of Tolkien's creations in OD&D (Hobbits, Ents, Balrogs, etc.).
In the end though, it come down to rather or not it makes the game better. In my opinion it does not.