Starting with r166, it will be possible to configure the shadow intensity per light by setting a new intensity property like so:
light.shadow.intensity = 0.5;
Some history about this issue: We could have added this change earlier but the previous implementation of MeshLambertMaterial was a blocker. Now that is uses per-fragment lighting and thus the same shadow code like other lit materials, shadow intensity could be properly added per light. The previous workarounds are not required anymore.
Related PR: LightShadow: Add `intensity`. by Mugen87 · Pull Request #28588 · mrdoob/three.js · GitHub