In a different kind of game, I would agree with you - but FFXIV has never been about flexibility. FFXIV is about a clear, unambiguous ideal we can realize and approach through rigorous measurement.
Content is designed with the following in mind: a minimum number of GCDs executed in a certain period of time with minimum player parameters. Once a group reaches the minimum HP parameter and the minimum healing parameter, the remaining parameter is the so-called DPS check. Superfluous healing and HP have no benefit so increased DPS is the logical choice; the most effective approach has been to outfit tanks with STR gear.
Experience reveals the minimum healing/HP parameters are low while DPS parameters are very high, so by the game's own design Fending gear is inherently less valuable than Slaying.
You could say superfluous HP/healing are training wheels and the transition to DPS gear is the mark of a PT ready to clear. This is not a very compelling choice, though. Tanks can go into content with DPS gear and clear it with the bar set that high from the start. You'll probably have an easier time clearing, since healers can tune their GCDs with tank HP X in mind.
So at worst you have completely useless endgame gear that clogs your drop pool, and at best you have endgame gear with an even shorter life cycle than the ilvl table it's part of.