All AS will shut down your natural test (except for a select few) so there is no argument against deca there.
My body also produces bile and HCl, but I don't want to inject that in my body, so that shouldn't be a reason to use Test over Deca.
Deca is rumored to activate progesterone receptors, and there is no conclusive proof that it actually causes progesterone induced side effects...only hearsay.
And about Ronnie Coleman's dosages... you and I would probably die if we used what he uses. Plus he uses other compounds like GH and Insulin along with the mega-doses of test.