Weight Painting and source (engine)

Now, this is a problem I’ve struggled with for some time and, at times, makes less than zero sense! I dunno if it was introduced by a newer build of Blender, if it’s some “feature”, or if it’s a bug but it’s making things impossible to work with.
See, if I’m rigging a model using weight paints, it’s some sort of absolute. 0.01 to 100 in value makes no difference on weights or how they are being pulled. It’s either “on” or “off”. Kind of like 1-bit. Anyway, I’ve been ignoring this problem for a long time, but I need a fix. Another important thing; when I import source engine models that are rigged, they correctly deform and are affected by the vertex groups. Weirdly enough, if I duplicate some faces from the model, they still deform, but if I add new geometry inside the mesh, it won’t deform correctly and has that “1-bit” problem.

An example:

One is the Pyro from Team Fortress, one set of faces duplicated from the original mesh, and a plane.