Roddenberry was probably trying to remain intellectually and maybe even philosophically consistent. The absence of the monetary system in his future vision should presuppose the absence of its main technique for self-perpetuation in the public consciousness (i.e. pop culture/media-consumerism/idolatry). I mean I don't think Roddenberry envisioned Black Fridays or Paris Hilton for his future humanity.
"The Spectacle is capital accumulated to the point where it becomes image." Guy Debord - The Society of the Spectacle.