This is documented behavior.
Indexes are not used with DESC.
Ver 4.X does, however support DESC with an index.

Grigoriy Vinogradov wrote:

>Bug Report:
>
>Version: 3.23.51-max
>OS: Windows ME
>
>Problem:
>Synopsis: Does not use index when ordering records on datetime field in
>the descending order, even though though this field is NOT NULL.
>
>I created the following table:
>
>CREATE TABLE Threads (threadID int NOT NULL PRIMARY KEY,
>                      forumID int NOT NULL REFERENCES Forums(forumID),
>                      discussion char(100),
>                      starter char(40),
>                      num_replies int,
>                      last_modified datetime NOT NULL,
>                      INDEX (forumID, last_modified));
>
>When I execute the following query with explain word I notice that index
>is not used:
>
>explain select * from Threads
>where forumID = 1 order by last_modified desc;
>
>If I want to select these records in the ascending order, than the index
>IS used.
>
>Thanks,
>Greg Vinogradov
>
>
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