It is interesting to hear that proponents of TEI are unsure about TEI advantages.
From my simple point of view TEI offers several advantages over e.g. pdf and html in web-publishing:

fast browsing; excellent  indexing; document  structure  is evident; nice and flexible layout; predictable cut’n paste;  displayed lower quality images can be linked to high quality versions. Now these advantages seem to some extent to be offset by a laborious production procedure. (TEI benefits should really be more explicitly outlined on the TEI home page.)

Regarding Indesign (CS5) I feel that the export possibilities (xhtml, epub and pdf) are unsatisfactory. Out of the box only pdf-exports delivers something that resembles the original print proof.
Having said that you should know that I have worked with software, computers and networks since the late sixties but that I am totally illiterate regarding layout, typography, printing and web-publishing. The reason for my interest in TEI is the result of browsing the internet for attractive publishing formats.

Professor emeritus Håkan Elmqvist
Department of Medical Laboratory Sciences & Technology
KAROLINSKA INSTITUTE, Novum, 141 57 Huddinge, Sweden