There are some terrible arguments in this thread. Where do people get the idea that everyone needs to be able to play every role? That might have been true in the past when you could end up in any role, but with a better method of queuing existing it isn't even close to true. I'd much rather master one role than be a jack of all trades.
You should call up a professional goaltender in the NHL and ask them why they don't get better at offense. Call up Tom Brady and say hey man you need to work on your blocking and tackling to really round out your game. Absurd. People should be able to play the role they want to, whether that means alternating between 5 or just focusing on a single one.