I would rarely need to add acid after adding an appropriate amount of calcium, so it would mostly be an either-or situation. Also, our local water has about 4 ppm calcium. Maybe that's why adding something that would remove additional calcium seems like a goofy idea, to me. But it also means I don't have any personal experience with the effects of adding phosphoric acid, either.
I worry that this argument sounds an awful lot like the "malt converts completely in xx minutes" argument. Well, your mash may convert that fast, but his doesn't. You may find that removing additional calcium by adding phosphoric acid doesn't hurt your beer, but that might not be true for the guy with 4, 10 or maybe even 50 ppm calcium in his water.
Really, I have no problem with adjusting the pH with phosphoric acid. Just the statement that it doesn't precipitate calcium without any evidence to support that claim.
The stoichiometry is relatively simple, so we can figure out how much calcium could be lost, which might be helpful. How much phosphoric acid are you typically adding?