James McMurray
First Post
One reason for not allowing the stacking of magical properties would be The Math. Assuming that you would need to trade something to gain the additional properties (ie, -1/extra property), you would end up falling behind on the expected numbers you should have, and would end up hitting less.
Not if your additional properties are in your off hand, as was mentioned in the OP. You can have your primary rod and also have your Rod of Corruption and Reaving +1 for instant annihilation of all minions unfortunate enough to be within 5 squares of another minion. Or any of a number of other combinations that aren't meant to exist.
The DMG doesn't mention anything about secondary properties that I remember, but AV does say they're not allowed, in a roundabout way. The Transfer Enchantment ritual lets you move magic from one item to another, but the target's abilities are wiped out in the process.
If you do allow it, I highly suggest against creating some sort of general formula. It's too open to abuse. Instead, look at every combination of powers as a new item altogether, compare it to current items, and find what level it should be.