Too each his own campaign. I personally think its an unfortunate house rule. Ring of Invisibility 20,000 gp. Cloak of Invisibility 20,000 gp. What's the difference? Same effect, same cost. Heck, the cloak seems more believable than the ring. Same thing for the Chameleon Ring. Ring of Wishes, Brooch of Wishes, no problem. I can't think of any effect that should be tied to rings only, but there might be some that should be.
Twelve levels is a long time to wait before you can get a deflection bonus. There's nothing wrong with allowing Wonderous Items that have a deflection bonus, particularly if you require Craft Arms and Weapons as a prerequisite.
One reason why this flexibility does not usurp the Craft Ring Feat is that Wonderous Items are limited by the item slots and slot affinities. Wonderous Items that don't take up an item slot or that don't match the slot affinity are increased in cost. That limits how many of these Items a player can use and the functions of those uses. Craft Ring allows two additional slots.
But, as I said, it's your campaign. Everyone has their own playstyle and the point of the game is to have fun. How do your crafter's like the rule, Rystil?