Speech level singing can get a bad rap these days.
For me, having my first lesson with a SLS instructor over 10 years ago, was true validation. Two main things happened for me:
1. I finally had a coach who was working my entire range at one time. Two and a half octave scales, going up or down by semi-tones, had me singing through five passagios in one exercise! I heard and felt improvement in my voice within the first day.
2. I finally found a teacher who encouraged vowels that didn’t sound classical.
Now, I’m not saying that forming classical-sounding vowels is a bad thing, I’m simply stating that I am not a classical singer, and had never encountered a teacher who allowed me to sound this way. I had been taught belting, and that just felt wrong. I now know that belting doesn’t need to be shouting. Healthy belting is done by mixing your registers.