As it is now with UL armor, the more decay it has the less protection it offers.
With the new armor skills it will make you more efficient using UL armor. The higher your armor skills, the less decay effects the defense of a UL armor piece.
There is no negative to this being implemented. It’s a win win for the player base.
I would answer yes, armor directly accelerates the acquisition of defense skills by allowing you to take bigger hits, longer. At a certain point your evader wont move very fast at all without armor that allows you to take the hits for hours on end for many high end mobs (since low mobs just wont give you defense skills)