While I think the concept of sliding scale damage reduction is a neat idea, I think this would be a poor way to implement it, because:

1) The way your propose to do it would just make mages spam ABS like spells until they improved. This is both boring and lame.

2) In order to spam these spells, they have to find the wand first. This can be time consuming in itself sometimes as a young mage. Once they have it, they'll hoard it for hours and hours and hours until it is spammed to max damage reduction, keeping it out of the hands of those who would actually USE it. Sleeks are a different story, of course, but this is still lame.

3) You can bet your ass you'd see mages using ABS constantly in this case whether they needed it or not for every fight as long as they had it available. More wand hoarding, more time spent gathering wands, and I think fighting classes would generally suffer due to this rather than benefit.

I've given this some thought for various reasons in the past, and I haven't been able to come up with a good solution. Sliding scale damage reduction based on level isn't really a good idea either... I'm at a loss.

--Nightshade