Model Matrix (transforms object space to world space)
View Matrix (transforms world space to eye space)
Projection Matrix (transforms eye space to screen space)
The model matrix is just the first of those three. Old fixed pipeline combined the model and view matrix together into the modelview matrix because it never required them separately. If your deferred renderer is defining the "position" in world space than you need the model matrix alone to do this transform. If you multiply a vertex by the modelview matrix you're going to get it's position in eye space, which probably isn't what you want.
All the built in gl_blahMatrix matrices are deprecated now anyway, you're supposed to be managing all matrices yourself on the client side, and then use uniform matrices to upload them to the shader.