Tether may be the most stable of the currencies, always maintaining their dollar parity value.

Is it really the most powerful? I think you have to look into Tether a bit more to understand that you may not want to put all your eggs into that basket. Also, why would you want to hold something that's only going to devalue over time? It's meant to be used as a transition currency, not really a hold. If you REALLY want to hold Tether, then just buy some USD.