Testnet coins are not incomplete neither full of bugs.
Testnet is just an alternative chain which is used for testing purposes.
Let's suppose you want to make a bitcoin wallet. You need to test bitcoin coming in and out of that wallet, signing messages and so on. There is no reason to buy real bitcoins for that.
Another example: you are making an ethereum smartcontract. You need to test as much as you like to debug your software. There is no reason to spend thousands of dollars in testing. Just go to a website and claim ETH testnet for free. You can test all you want for free without spamming or congesting ethereum network.
Which investor would buy something which anyone can redeem for free?
Testnet coins are not meant for investors, but to developers.