I think the reason a lot of guys use steriods, is that they over-train when natural (many don't eat right either, but that's another thread).
Then what a lot of guys do is they go ON, and proceed to train even MORE and therefore still have some issues with overtraining while ON as well.
I am starting to have a lot more respect for the Hardgainer methods, books like Braun, techniques like HIT, etc.
If you are not allowing ample time for full recovery and growth from youyr previous workout before working that same muscle again, you ARE lessening your gains. Do that too much and over time, and you will be overtraining.