Keep in mind...
This is NOT about how great singers sing (and phrase a song). That's a different discussion.
It's about how a song is WRITTEN using what will be "vocal rhythm" to get a lyric across.
Really though, when you think about it, those two things are very closely related cousins.
Take the same song that was a hit for two different singers and they can phrase it differently. Take any of a bunch of Willie Nelson's songs as the example here.
Yep... it's very hard to get the phrasing and rhythmic feel of a lyric just from the written words. It's then up to the singer to interpret the song as they best see fit. If they have a demo, that makes life simpler.
Regarding the concept that the rhythm of the singing is an integral part of making the song what it is..... oh yes, absolutely.