Why would you do it for free when you can get paid by the potential projects? If you want to build your portfolio, you can simply make your own personal projects instead of working for these ICO scumbags for free. You'd get plenty of messages from the scammers to work for their project and they'd promise you to pay the tokens in return. In reality, they will use your code to show they're legit and run a scam ICO. Also, you'd have a good demand around if you can actually write a good smart contract on ETH, there are very few good developers around who know to code.