Although people don't get 'extra time' in the world of work, there are few scenarios where you are placed under timed conditions in anywhere near the same way you would be in an exam. And where these do arise it is unlikely someone who has difficulties would consider themselves appropriate/wish to work in such a role anyway, they're more likely to pick a role more suited to them.
Generally, in the workplace (and everyday life) you learn to adapt and manage your difficulties. When you know a certain task is going to take you longer, you give it more time and might stay a bit late at the office, maybe work through lunch or get other things done quicker to give this more challenging task more of your day. If you cant remember something, you can ask a colleague to help, or ask the person on the phone to bear with you whilst you double check. You can use a computer to write, software to help you, you can look up spellings, all things you can't do under exam conditions.
Exams don't reflect on real life and the situation is something you're unlikely to face in the workplace. It is likely your deadlines will be weeks, days, or 'by lunch', rather than in one or two hours. Yes you need to be able to get things done in a time frame but a larger one with resources to help you. Yes you need to manage your time but you'll be much more experienced in what you're doing and how to do it. Questions on an exam could be anything.
Taking someone like Steven Hawking is he had has his condition whilst at school... He's incredibly intelligent and obviously would deserve high grades. He would get extra time as he cant just quickly sprawl his answers like the rest of us. Extreme example as its a very severe disability but if we took extra time away from people such as him we could discourage/ fail to help the growth of some minds and overlook real intelligence that would be an asset to certain fields of work.