But test mastercoins were released together with real mastercoins simultaneously. As far as I understand, you can do the same thing with test coins as with real coins. So, if there will be an exchange trading them in the future, they WILL be useful, and you can trade them back to bitcoins or whatever and buy something with it.
Still not different from testnet bitcoins.
There are so many other cryptocurrencies with hardly any innovation in them, but still they have some value, because you can exchange them with other currencies.
These alts have people advocating their use, which results in people speculating they have a chance to catch on (and they still capture only a small percentage of Bitcoin's market cap). For test coins you have no such thing.
Actually, what you're doing right here is a form of advocacy that testcoins should be used. Keep doing that and you might fulfill your prophecy.
I just wanted to mention this possibility, because ultimately, test coins might lower the value of real coins because of more supply. So it might be good to destroy them at some point, before it's too late. And of course let people know early enough about that action. I think this helps, because people can actually treat them as test coins and therefore also 'waste' them for testing purpuses.
Test mastercoins are effectively a clone of mastercoins. If you fear that mastercoins can lose their value due to the existence of a clone, and clones are trivial to create, why do you think mastercoins can have any value at all?