The MHX importer has an option for importing body shape keys, to preserve volume and to add some muscles when the bones are moving.

Consider the MakeHuman mesh, imported into Blender with the option for body shape keys unselected. When the arm is bent, the mesh collapes into itself at the inside of the elbow, in a quite ugly way. Even if you can evade this problem to some degree if you paint weights more carefully than I have done, this is a quite generic problem, not specific to MakeHuman or Blender.

Now driven shape keys come to the rescue. The shape key BendElbowForward_L is not pretty; it looks like our character has a serious health problem. However, this shape key is not seen when the arm is straight. Instead, it is connected to the rotation of the lower arm bone LoArm_L along its local X azis. The influence of the shape key varies linearly from 0.0 at 0 degrees to 1.0 at 90 degrees. Now when we bend the arm, the shape key cancels the collapse of the mesh, making the elbow deform much nicer. Perhaps the shape of the elbow is still not anatomically correct, but it looks a great deal better than before. As a bonus, a little biceps bulge has been added to the BendElbowForward_L shapekey, although it can barely be noticed.

Caveats

These volume-preserving shape keys have two limitations. They are not really up to the artistic standards that you would expect from MakeHuman, because they were not made by an artist. More fundamentally, the shape keys where edited on one particular base mesh. For characters that deviate very far from the base, the shapes should be different. Just consider the difference in bicep bulge between an anorectic person and a bodybuilder. Both these issues will hopefully be addressed with the pose engine.