The rfc's http://www.w3.org/Protocols/rfc1341/7_2_Multipart.html on multipart/alternative specifically deal with the display preferences;
In general, choosing the best type means displaying only the LAST part that can be displayed....In general, user agents that compose multipart/alternative entities should place the body parts in increasing order of preference, that is, with the preferred format last....From an implementor's perspective, it might seem more sensible to reverse this ordering, and have the plainest alternative last. However, placing the plainest alternative first is the friendliest possible option when mutlipart/alternative entities are viewed using a non-MIME- compliant mail reader.
Scott
