During the Chazzan's repetition of the Shemoneh Esrei, one is generally required to pay absolute attention and avoid learning etc.
What about when the Chazzan is reciting piyutim during his repetition? Need one focus on his words, or can one learn or engage in other activities? After all, piyutim are not required (the repetition's valid if they're omitted), and there's less of a problem of berachah levatalah.