Isn't full ROM important also? I think both can be good as long as you understand why you are doing what you are doing.
I don't keep tension on the eccentric when I am training for power. I fully extend my arm, hold for a second, and then without rocking or leaning back I blast the weight up.
When I train for hypertrophy, I keep tension on the concentric. I go really slow down and up while squeezing the hell out of my bicep at the top of the movement.
Both are affective. It all depends on what your goals are.