I don't agree, but I DO see the point. I would only offer that it depends on WHEN you hear him. While you can always tell it's "Ronnie James Dio", his voice is a bit clearer, a bit cleaner on the Rainbow records. Live, and I'm specifically referring to the German shows from '76, some of his vocals on Catch The Rainbow, Stargazer and Mistreated in particular are very strong. When he went to Sabbath, there was a slight... roughening of his voice, but on certain songs on Heaven And Hell - Die Young, Children Of The Sea, Walk Away - there was still that clarity. With Mob Rules, he started to sing in more that gruff-y, metal voice, and that continued on to his solo stuff. The later Sabbath records (and teh Heaven And Hell record) TO ME suffer, in that it seems that Dio was trying to be that "metal icon" and Iommi was the "Riff Master General" and it became very "same-y" to me. Perhaps understandable given the amount of touring he did (whether you like it or not, his voice stood the test of time about as well as any big-time vocalist.
TL;DR: I agree with that in the later years of his career, I agree with that less and less as you go back in time.