Last year I set up a Cardano testnet as part of the Plutus Pioneers program. I'm now using my Daedalus installation to experiment with the cardano-cli, which I am finding much easier, with the added benefit of knowing that my transactions actually work in the real world. The transaction cost so far is trivial. Is there any reason not to use the mainnet for R&D, learning, and experimentation? I can't imagine I'm stressing the network, and I'm putting only my own ADA at risk (little 2-5 ADA transactions). I can imagine I could clutter up the blockchain with pointless metadata.
Closely related, is there a way to use Daedalus to talk to the testnet?