I can only say that to view magical items, whether weapons or otherwise as simply abilities with limitations on usefulness is to rob Glorantha of one of its strongest themes, that of magic. In an extreme magic from the Gods themselves is a bit like quantum physics, past a certain point the normal rules need no longer apply. As long as it works for you in terms of game theme, style or whatever, I do not see that the rules need be an issue. In fact so far I have not seen any rules that say DO NOT DO IT LIKE THIS, more the opposite in fact, certainly in tone and I believe intent.
In the description of the bag of wind magic item it clearly states that it is not known what this does and it will be worked out later. To simply limit yourself to saying this has a XYZ ability or feat at level 12 is very dull. Far more interesting and fun to generate a description of what it is and how it can be used. It introduces a whole new tier to the character who ownes its game life.
Certainly if you do not feel confortable with something don't use it, but I would much rather see lots of posts suggesting new ideas than to have everything reduced to the douer ability 12 regime that seems to be circulating.
If you allow a magic item to be different and it proves to be unbalanced you will soon know. On the other hand if it is too weak you can always let the character discover new powers within as the game progresses.
'There is always another way'
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