Thank you Wet Boots, cjohn, mdirr and Al. Your comments help. Texas's code says that the purpose is to ensure 'accurate testing of the backflow preventer.' A lab engineer/tech at fccchr said on the phone that a non-constant pressure and so all the resultant cycling of the backflow parts tend to promote a higher fail rate, hence the recommendation in general and requirement for new installations in many states.
Whether it is worth the thousand dollars or so to move the master valve to the discharge side on old installations seems a guess to me. Liability issues are big here at the HOA, so with a stretch of reasoning, this might justify the expense.