The following is copied from pp. 5-6 of "The Cantor's Companion".
I think it addresses Sophie's concern, and it struck me the first time I read it as being nicely phrased.
The rhythm of Carpathian chant is essentially that of �sung speech�. The value or duration of a whole note, half note, quarter note, and eighth note are in proportion to each other. A half note is sung half as long as a whole note. A quarter note is sung half as long as a half note. An eighth note is sung half as long as a quarter note. However, the cantor is to sing as if speaking the text, in neither a rushed way nor a long and drawn out manner. When the singing moves back and forth between the clergy, lector, and faithful, the chant should flow as in a conversation. This is the reason for the lack of time signatures in notated chant; the rhythm of the singing is natural and based on the text, rather than a fixed, regular �beat.�