Diehard, on it's own, isn't all that awesome, true (doubly so if you impose some kind of nerf based on a strange interpretation of the rules, be w/e). That said, it can actually be a fairly awesome feat, in the right circumstances.
Delay Death.
3rd-level Cleric spell, touch range, 1 round/level. Makes the target immune to dying when below -9 HP. Normally a craptastic spell, since it lasts no time and doesn't even stop its target from bleeding out. With Diehard, it becomes kind of decent, since you could be active and fighting for a few rounds after you would otherwise have died, kind of like Tryndamere.
On the other hand, if you have Diehard and
Persistent Delay Death, then you're totally immune to death from HP damage. Of course, this isn't quite an ultimate defense, since there's a number of things that can kill you anyways... Disintegrate (which would ordinarily kill you through Diehard), ability score depletion, level drain, and death effects. Also, being a buff, Delay Death can be dispelled as normal. Even so, Death Ward is enough to protect against most of the other things that could kill you, and by the time you get into the levels where you need to seriously worry about them, Death Ward is already on the standard list of pre-combat buffs.
Now, Diehard doesn't really fit into the build of a DMM Cleric, who has MUCH better things he could be doing with two feats. But if you ARE a DMM Cleric, and happen to have a meatshield that likes Diehard, then you're in business... after all, in this situation, Diehard is far superior to just about any other feat the poor DMF could be taking.