The way I see it, rapping can be singing somewhat in a melodic way.
If James Brown is considered the Godfather of Hip Hop because of how he sang over a beat, if Bone Thugs and Harmony, Nelly, Ja Rule and 50 Cent can sing their hooks and be a considered rappers, I say let the new dudes sing songy their hooks.
The truth is, being sing songy makes more money and you can do more musically. These new rappers, all their favorites could carry a little tune.
I see Hip Hop going the way of Reggaeton, where "rappers" make nothing but sing songy songs.
I blame RnB and Rock becoming "old folks music" for the reason rappers are now yodeling on the track.