Really, poets need a feel for musicality every bit as much as lyricists. Even those who write free verse -- one has to have a feel for the natural rhythms, the sound of the words. This is one reason (of many) why there is a plethora of both bad poetry and bad lyrics.

I wonder if I could use the word 'plethora' in a song.

On the use of thesauri and rhyming dictionaries: the great thing is that one can, while looking for the right word or rhyme, come across a word that sends their thoughts in a new and different direction. It is an aid for 'thinking outside the box,' if you'll excuse the cliche. Sort of like a co-writer!