That's a good question.
Nothing stops you to deloy "in production" a dApp running on Rinkeby and it will work and be free however it would definitely be less secure than the Mainnet network.
The Mainnet network is very secure for the following reasons:
- a large set of miners increase decentralization and reduce the risk of 51% attack. Miners are incentivise by block reward (in ether which has a real monetary value).
- RinkeBy network works with a Proof Of Authority consensus, meaning that the network is run by a set of validators. Those validators could decide to not mine your transactions, validators could decide to stop the network, etc...
- Ropsten is more closed to the Mainnet because it works with Proof of Work, but there is less nodes running and I imagine most of them are run by the Ethereum Foundation and a few organizations in the space. So it's less decentralized again.
It's all about having decentralization and security often at the cost of scalability and important transaction fees.
That's why there's a lot of debates and research around sidechain where by design, some assets requires less security than other and could be transacted on a less secure, faster, cheaper network called sidechain.
The sidechain could be a testnet network like Ropsten and Rinkeby.