Yes, it would have been. But I'm sure you won't enjoy it as much when the bank rectifies the mistake and debits your accounts accordingly. Unless you withdraw everything you've got and disappear from the face of the earth or stash the stolen cash somewhere no one can find, it won't be that easy to steal from a money machine.
The flip side of excess cash at withdrawals is no cash dispensed and your account has already reflected the deduction. When you complain to the bank, they can trace the undispensed cash and return the funds to your account. The same goes for excesses in cash dispensed. The bank will be able to trace the accounts and the actual cash dispensed on each transaction and will take it out of your account.
It may sound like a great idea to request 100 and get 200 in return, but since the money is tied to your bank account when you use an ATM, it is still your money that you are taking and not just some free handout.