Defence should just be the measure of your character being able to do anything defensive. If you suck with a shield, you won't block much with it and it will probably hinder your character in combat more than help it. What does this mean? If your defensive skill was at a "high level" and your shield skill was "low level" might mean that you defending all around worse making you unable to successfully defend (or defend well enough with a shield equipped to have the MUD recognize it as "successful").
I find that the skill system (especially at high levels) will only reward successful or critical hits to improve the skill.
That said, if you are defending yourself poorly with a shield, the number of "successes" or "critical defends" would be much lower with the shield than without the hindrance of a shield.
If this is the case, wearing a shield vs wearing no shield would make you less likely to get improvements.
I would assume that the better you get with a shield means that you would be able to get more of these "critical defends" and raise your defensive skill.
This is all just speculative over my years playing here. I could be wrong. But it does make sense doesn't it?
The right man in the wrong place can make all the difference in the world.