I have no idea why you think 40/0/0 is better than 20/10/10.
40/0/0 means I only spend one item slot for 40 skill. 20/10/10 means you have to sacrifice 3. That's a WAY higher cost. It also means you can't find one random item and have your spell skill complete; you need to 3.
Putting it on all THREE means you have to commit to the strategy; putting it all on the weapon just means you need to get lucky.
40/0/0 means I only spend one item slot for 40 skill. 20/10/10 means you have to sacrifice 3. That's a WAY higher cost. It also means you can't find one random item and have your spell skill complete; you need to 3.
Putting it on all THREE means you have to commit to the strategy; putting it all on the weapon just means you need to get lucky.