Tying defensive effects to weaponskills that are a part of your rotation either result in them always being up, like Heavensward Delerium, or having no proper control over their timing and usage. Both of those situations seem boring and less than ideal.
Tying defensive effects to weaponskills that are not tied to a set rotation but are based on cooldown just makes them tend to be used as another damage ability, foregoing the defensive aspect for the dps.
A small level of passively applied defense is fine and already exists for each of the tanks, but trying to make the prominent defense abilities like that would not work out well.
The closest thing to this that I could see maybe, emphasis on the maybe, working would be removing the defensive buff from the Tank Mastery trait and rolling that into some sort of weaponskill rotation or another ability that has to be managed and kept up similar to how some of the dps buffs like Darkside and Surging Tempest (aka Storm's Eye) are kept up.