It’s a stamina bonus and a health bonus. So it’s not just 5 stamina, it’s 5 stamina and 28 health. And while vicious waistguard is obviously comparable, for one the epic variant of it is currently uncraftable. For another it costs eldarium. And lastly it’s a particularly stat efficient piece and other pieces of the vicious armor don’t compare as well to stygian commander.
Ignoring the stamina bonus makes little sense, as stamina is generally a valuable stat, whereas health is always a valuable stat. Stamina usually being a much more valuable than follower damage/concussive damage that Theurgical set gives on top of health, which both do.