And if the shop sent it to a hydro facility that can't do the + rating, you have a legit beef with the shop. + ratings CAN be recovered by passing, and the shop should send it to a hydrofacility that CAN do the plus rating. (Personal opinion: If I ran I a dive shop, I would NOT use a hydro facility that can't do a + rating.)
That's part of what I was getting at. If a customer has a tank with a recent hydro date, it somehow then fails viz, gets put through a new hydro, then tumbled (and what, then hydro'd again post-tumble?) and viz'd, and comes back to the customer without a plus rating (even if the tank wasn't plus-rated before, it's something the customer may have wanted to request), it just seems to suck for the customer.
Also, if it fails hydro on that third time, who's to say the stress of the "unnecessary" hydro didn't push it over the edge? Sure, maybe that's unlikely, and you can say unsafe is unsafe, but I assume there's a reason why tanks aren't hydro'd every year, or done ten times in one sitting just to be safe.
In the end, it seems to be a question of whether by doing so, the shop is taking an action that is not only unncessary, but may also have an impact on the life of the tank?