That's true. But Williams does enough other things well for his footskills to be something that the team can still win with. If he is surrounded by good kicks, it's not a problem. Just as Heath Shaw isn't a good kick, but he's still one of the better general backmen in the competition for how well rounded his game is, Williams while he has a slightly different combination of strengths, his usefulness down back as with Shaw by position is beyond doubt.
It's like saying Nat Fyfe is a dud because he's a poor kick. He's a dominating contested ball winner, stoppage player and is a massive threat overhead. It's certainly preferred that everyone can kick, just as it's preferred everyone can win their own ball. There is more to AFL than just competence or otherwise in just one attribute. It's a team game for a reason.