you can do quite a lot with models in halflife, in essence models in halflife can provide the detail in much the same way as is performed in halflife 2 (albeit with the many limitations that come with a 5 year old engine!)
If you were skilled enough you could indeed create a new model for mr freeman, though you would only be able to view it in multiplayer/on a camera. You could even change the animations if you wanted (thus the model would cease to be just a 'skin' of the existing gordon model).
I tend to use models where a high number of polies are needed and the use of world brushes (something created in hammer) is out of the question. In my last few maps I created wooden bridges, telephone poles (so many could be repeated with good detail), AA guns, backpacks. Anything that I needed really, its a fun addition to the engine and can really bring out a map if you use them well.