What about the fact the dps can and do take unnecessary damage, which takes up healing which would normally go to a tank?
How would stacking hp be any better than stacking mitigation? All that hp stacking does is buy you a bit more time on F/T heavy phases. In every other case, hp stacking would be *worse* because you're relying on having your hp constantly refreshed by your healers rather than simply requiring less healing. With hp stacking setups, you're going to require more attention from your healers, which means that, if the DPS are eating it to the face like the baddies they are, the healers will be even more overloaded. As a mitigation stacked tank, you're functionally allowing heals on you to count for greater effect (since you have fewer hp and they absorb more unmitigated damage on a point-for-point basis).
eHP is only an effective metric for gauging the time that you can survive without getting a heal. Once that time frame is above 10 seconds or so for the relevant content, it really doesn't matter *how* long it is, because it's long enough that, unless your healers are simply *ignoring you*, they'll be able to bring you back up in time even if the DPS are being idiots. If you focus too heavily on increasing your eHP at the cost of mitigation, you're really only forcing your healers to dump more resources into you for no real benefit.