Same as Marko here.
I'm using LWPolylines instead - since they are alot more flexible in editing (and like you pointed mline has limited....).
Also when using mlines and you have to send your dwg for consultation, the architect/engineer/colleague guy might ask how to explode that geometry, beacause he can't work with such objects.
I've had situations where I had to explode/burst blocks, ungroup groups and resend the dwg, because the other guy had limited CAD knowledge and just doesn't listen to "Hey just use EXPLODE and UNGROUP commands".
So for walls I use a bunch of curve/geometry manipulation functions and techniques (to be more comfortable):
Multiple offseting, and capping or just something like
this. Drawing some rectangles that partially overlap, then use Lee's
outline routine.
Maybe the only useful thing for mlines would be to draw roads with sidewalks for urban planning, in the end use something like
this and fillet the curves at the mline's intersections.
On the other hand I won't mind exploring MLines with lisp.