Is tension a bad thing?

Yes and No.  

There is tension that is definitely bad which stems from muscles fighting one another, and manifests as constriction, shortened range, no volume control, distorted vowels and diction, etc.   

But, there is tension that is good. Remember, we are talking about muscles in singing. These muscles, in order to vibrate and make different sounds, have to be of a certain tension.

Think of it like the strings on a piano.  To make the low pitches, the strings are long and thick and have less tension. To make the high pitches, the strings are small and thin and have more tension. Our cords are like this as they are adjusted for pitch. There is a tension that exists, and it is necessary.  

When the muscles play nice, there is this healthy tension - plus the muscles being under tension also means that muscles develop which equals bonus vocal fitness!