Collins, Derek. 2004. Master of the Game: Competition and Performance in Greek Poetry. Hellenic Studies Series 7. Washington, DC: Center for Hellenic Studies. http://nrs.harvard.edu/urn-3:hul.ebook:CHS_CollinsD.Master_of_the_Game.2004.
21. Conclusions and Prospects
For Sealey, and many before and after him, Phemius and Demodocus in the Odyssey represent the poets (aoidoi) who compose while they perform, while Ion, the rhapsode (rhapsôidos) featured in Plato’s dialogue by that name, represents the opposite extreme of the largely recitational performer. The case for creativity among rhapsodes has not been made easier by the prejudices of Plato (as evidenced in the Ion) and Xenophon, who ranks them among the stupidest of men (Symposium 3.6, Memorabilia 4.2.10). For Plato and Xenophon, although rhapsodes may recite Homer’s words correctly, they simply do not know what they mean.
The verb κοσμεῖν ‘embellish, adorn’, as others have noted,  elsewhere in the Ion refers to adornment with regard to clothing (530b5, 535d1), and in itself cannot be translated as ‘improvise’. However, given the improvisational skills of rhapsodes that we have seen, I suggest that Ion’s “embellishment” of Homer be interpreted broadly to include the range of rhapsodic performance techniques: mimetic and gestural elements, vocal range, and especially improvisation and modification of verses. Verbal improvisation against tradition was integral, if admittedly not exclusive, to the popular appeal of rhapsodic competition in performance, and we must see that such competition was essentially a poetic game. The master of that game, like Ion, was the one who most deftly displayed his rhapsodic abilities in live performance.