Training the voice is a complex work of opposite forces, and it does not work with the simple opposition of opposite muscles. We can think in a more complete and refined way putting the opposition on the functions so to work in some parts or in all the muscular chains and obtain not by luck all the different possibilities of voices and not strain or remain with a too limited voice.
If you are a boxeur or a sprinter will you train just in a relaxed and smooth way, as for example doing only Tai Chi? Relaxation can be useful and even fantastic as Tai Chi, but it is one of the tools possible and useful in voice training and it does not serve all the required purposes for professionals and performances or certain styles. Music and repertoire are made of different rhythms and accents and strenghts and if we do not train the system voice in a rational and precise way to be the perfect image of the music we are going to perform we will have a problem. Of course how to do that and from where and by what passages develop strengths and speeds requires great care and wisdom as in every training, but the answer always smooth and legato and relaxed is not the answer correct.