Carl sang it too for a while. Perhaps Dennis sang it in 1973 and 1974 because he wanted to do a rock song. Perhaps Do You Wanna Dance was not to his liking at that pioint. It is odd. Someone should ask Jardine.
Well Carl took over the lead vocal on "Rhonda" when they introduced the new, slowed-groove boogie-fied version in drop-D tuning (early '71). By 1973, the tempo had picked up and Carl probably handed it over to Dennis so Denny would have a rocker to sing in the show. Sometimes they would sing it together. But Al sometimes sang it in '73 as well (as on the "In Concert" LP).