I've been able to consistently reproduce this bug.
If I make a new session and go to import a Source mesh, the default armature mode is "Append to Target".

If I import a DMX, the next time I go to import a Source mesh, it's set to "Make New Armature".
If I change it back to "Append to Target", import a DMX, and go to import another mesh, it's at "Append to Target" again, and will stay there until I change it myself or make a new session or something, haven't tested that thoroughly (except making a new session).
This is regardless of whether the DMX has multiple parts/objects or only one.

But on the other hand, if the first import is an SMD, the next import will default to "Append to Target", as expected. If one imports a DMX after an SMD, it also defaults to "Append to Target".


Observed in the 64-bit Steam version of Blender 2.74, using version 2.5.2 of the Blender Source Tools on a 64-bit Windows 7.