Chantry would not approve of Joining because of two reasons:
1) Joining ritual is some kind of a blood magic ritual. What Warden Recruits drink = Archdemon Blood + Darkspawn Blood + Lyrium
2) Because the Chantry is full of fools.
Grey Wardens do the Maker's work much better than the Chantry, Seekers or Templars did in their entire existence. Maker himself banished Old Gods from heavens and imprisoned underground. Old Gods are referred to as "Dragons with wicked eyes and wicked hearts" and we know that an Archdemon is a corrupted Old God. Grey Wardens sacrifice themselves to kill them. So basically, Grey Wardens = Those who stand before the corrupt and wicked and do not falter. Dagna refers to the Hero of Ferelden as "only light in those dark times". That is what Grey Wardens are, lights in shadows. They drink the blood of darkspawn and Archdemon to fight them. They are also peacekeepers : "In Peace, Vigilance". There is Maker's will written in their bloods, proof of their sacrifice. Grey Wardens sacrifice themselves while fighting the corrupt and the wicked while the Chantry, with all their self-righteousness, judges everyhing and everyone that doesn't fit their description of Andrastian.
Blessed are they who stand before
The corrupt and the wicked and do not falter.
Blessed are the peacekeepers, the champions of the just.
Blessed are the righteous, the lights in the shadow.
In their blood the Maker's will is written
these verses fit Grey Wardens much better than Chantry/Seekers/Templars. There are great secrets about Grey Wardens and I'm sure the next game will focus on Grey Wardens, Blight and Mythal's relation with Blight. That is why I want an experienced Grey Warden (like Warden Commander of Amaranthine from Awakening) as PC for the next game, who at least saw Architect, learned that the Architect has started the Fifth Blight in a failed attempt for making Old God Urthemiel his disciple. That would help the story greatly.