Yes, well, if a spell fires on each successful hit it pretty much makes a weapon massively overpowered. Imagine putting something like On Hit: Cast Sunburst on a Divine Mace or something - you'd only want it to fire 5% of the time because if it fires every time it makes the weapon hugely overpowered. I'm a bit disappointed with that property then.
Chewy, where's that thread you're referring to? There are a million threads there with "item creation" in the text at the Bio forums.