One of the benefits of communal prayer is the ability to say K'dusha in the repetition of the Amidah. We also say "Kadosh, Kadosh, Kadosh" (Is 6) as a preparatory component of Shma, and in U'Va L'Zion (sorry, no hebrew keyboard).
If we posit that angels do not have free will (or at best, the ability to make judgement calls on how to fulfill their purpose), then how are our references to their praise of HaShem interpreted by HaShem. It would seem that we are taking a step backwards, and not evidencing our free will to perform Avodah Sh'balev. Does placing the emphasis on their Avodah somehow imply that we should aspire to their form of worship, which does not seem to be given freely?