How'd you come up with DC 12 + Enemy's attack bonus?
In my opinion, rolling more is not more fun.. Player or GM.. As a player, I'd definitely rather not do more math.. I also can't see how this makes it easier to keep track of reaction powers.. Whether the DM tells you what the monster got to hit you or you tell the DM you defended the monster attacks seems to be about the same.
In terms of DM workload, I don't think rolling has ever been the big thing that's heavy on DM workload.. It's world building, encounter building, tracking enemy HP/conditions, etc.. The bookkeeping is the hard part, not the part where you roll the di, add a number, and ask the player if that hits a defense.
I guess I can't really think of many downsides, but don't really see the upsides. Essentially instead of you rolling a di and saying, did an 18 hit your Will.. You'll now tell the player "Roll your will defense, DC is 18".. All seems about the same imo.