It's four or five purples for each blue. And you definitely do not have to get every single schematic as a lot of them are not worth much.
- cunning and ANY defensive stat: no cunning tank
- any redoubt with anything but shield: tanks do not want any other stat
- any non-redoubt with shield: again no tank wants any other stat and non-tanks don't want defensive stats
- any presence: you're better served gearing up your companions than gearing yourself to buff your companions
If you get these while going for more worthwhile schematics, so be it, but they won't sell.
I like the cunning redoubts! Hear me out before you scream...
In some cases the defense is enough to reduce damage and therefore you last longer. its great if you are like a sniper or the like with DPS in that it allows for you to get a greater damage avoidance. Basically in a nutshell, it is great if your 'role' is to provide suppression fire and keep mobs from concentrating out on a your tank.