While I agree with you in principle, I do disagree on a few specifics.
Any character capable of self-healing might be better off with mana regen items.
I cautiously agree. Normally a Vlemish Faceguard and Scale Helm in conjunction is enough to meet a character's mana needs. If it isn't, however, I find the heart of life is a better investment than upgrading to those of those extremely expensive helms. It's in the same price range, and offers you a versatile tool. You don't always want it, but sometimes it fulfills your needs just right even if you do have other forms of healing available.
If you're playing on a map with multiple crystals (Exile and Mandala), the need for an HoL is somewhat alleviated. Very frequently, on those maps, you can port/run to the nearest crystal and run back to the front with little lost time.
If those flags are being held by an enemy demigod, heart can be critical for flushing him out. After you beat them the first time in battle, they'll run back to their crystal and charge up while you're trying to take the flag, meaning you basically have to win two fights in a row against the same demigod (and this is before we even consider flag locks...) to actually capture it. Heart allows you to regenerate while he's regenerating.
I totally agree about the unclean beast. Don't get heart if you're up against him (unless you happen to be Oak). I disagree about Regulus. Snipe has a fairly long setup time, so if he starts preparing to snipe the moment you activate your heart, you'll still get most of your health and mana back before you actually get hit. In fact, heart has saved my life from snipe a couple of times by giving me just enough health to survive it. I'd also disagree on Rook because the trebuchet hitting you is something that doesn't happen very often. It might only happen once every five or six times, and that's an acceptable fail rate for an item this good.