Specifically, end-nodes typically extract data from MIME and use it within the local context of that application *ONLY*. The encoded data is RARELY used inside of other applications, and the extracted data is typically used for this purpose, when needed. On the other hand, domain names are frequently cross-populated among applications, and as the argument regarding clipboards shows, the encoded form will be the norm.
Furthermore, MIME and its constituent application protocols are being extended so that binary data *CAN* be transferred without being encoded first.
--Paul Hoffman, Director --Internet Mail Consortium