Will you be mining Bitcoin Testnet 5 when it is released?
Probably not, because ASICs would be needed for that, and I don't have that much mining power. The starting difficulty was recently changed from 0x1d00ffff into 0x1a0fffff, which means, that CPUs won't have much chances there, because it is the difficulty around 1,000,000, which is too high for any CPUs. Fortunately, there are ways to lock coins on arbitrary difficulty inside scripts, so it can be used to test mining instead, and for development, not that much computing power is needed anyway.
or because you want to contribute to a working network?
There is a working network called regtest, where nobody would dare to list on any exchange, because it is designed to be run locally, and it is too unstable to be run publicly, without additional protections. And Proof of Work scripts work there as well, as they do on mainnet, so it is sufficient to test new solutions in that way.
Does Testnet having a market really make that much of a difference to miners?
Most altcoins are created mainly because people cannot mine BTCs directly. If not that, then there would be much less altcoins, than we can see now.
It makes a difference for developers, willing to test their code, not miners. People engaged in mining are usually happy, that they can sell new coins for BTCs.
