I support the idea that he should serve some purpose after a player has gotten the plan. I don't know if making him drop a random key is necessarily the best improvement, but its certainly better than the current implementation.
Yup - it's a good idea. Andrew just added this! Should be in the next PTR patch.
It doesn't impact the drop chance of the plans; it's actually a bonus. :)
(The plans can still drop even if you have them, by the way. The game currently doesn't have a way to check if you have the plans or not. That might be something we work on later, but for now it shouldn't affect you either way -- you can still farm for the plans and/or for the random keys.)