Eccentic muscle action is incredibly important. Concentric only movements do not develop the stretch reflex. There is no way in hell you will be able to move heavy weights without teaching your muscles how to develop and utilize kinetic energy accumulated through the stretch shortening cycle. I am not saying doing negatives is a good thing, because they will not make you stronger. I am talking about maximizing force production using optimal (i.e. fast as shit) eccentrics.
Also, your muscles have mechanoreceptors that send signals to shut down contraction when there is too much stretch/load/pressure in a muscle. Fast eccentrics train these recpceptors to not engage so that the above mentioned kinetic energy can be used in the development of force.
Slow negatives will make you bigger because they do significantly more damage to muscle tissue than concentric and isometric muscle actions. This is not the goal for strength/power athletes. The goal is increased inter/intra-muscular coordination so that heavier weights can be lifted more efficiently. Recovery is vital so that more/heavier work can be done more often. Negatives take the standard rule of 72 hours between intense workouts and pisses all over it.
Every method has its place in training, depending on your own goals and weaknesses, but using negatives on a regular basis or using just concentric only exercises on a regular basis is not the best idea.