So we are talking 21k healing just to the tank. I can do that in about 15 seconds. That means that of all the additional adds around us it will take according to the chart 11k damage by one of the DPS to pull ONE add off me.
Did you not read what I said about healing aggro being diluted across all enemies, not to mention the standard 15% reduced threat that Sages generated via healing (unless you just didn't take that talent, which would be remarkably stupid of you to do)? You're also making the rather strong assumption that every single bit of that healing is going to be required. Overhealing generates no aggro whatsoever. So, unless your tank actually *needed* every single drop of that healing, you didn't generate that much threat.
Assuming you threw out 21k actual healing (which means no overhealing) over 15 seconds, you just generated a whole 600 TPS (21k healing * .5 healing threat modifier * .85 Sage healing threat modifier / 15 seconds), and that's assuming you got that lucky chain of crits you talked about, even if it's not really all that likely (even on HT, you're not likely to get a crit off of every tick since Rejuvenate just increases crit chance by 25%; with a 35% base chance to crit, you would have a 12.96% chance of that actually occurring). 600 TPS on a single enemy is a complete joke: any DPS or tank is going to *quickly* plow through that tiny amount you're generating.
Now, if were talking about a trash pull, you have to factor in the fact that healing threat is diluted across all enemies you're currently in combat with (which you seem to have ignored since I've said it multiple times). This means that the 600 TPS you're generating is only actually 600 TPS if you're fighting a single target: for 2 targets, it's 300 TPS, and for 3 targets it's 200 TPS. For the first pull of Maelstrom Prison (which is, iirc, 9 targets), you're generating a whopping 66.67 TPS on each target. A DPS or tank is simply going to have to *sneeze* at a target to pull threat off of you, assuming you ever had it in the first place.
With the guard on me it will reduce that threat by 50%. VERY necessary if the tank whats to scoop those adds up with an AoE taunt.
You obviously don't understand *either* of the mechanisms you're talking about. First off, Guard reduces threat generated by 25%, not 50%. Secondly, if the tank is using their AoE taunt, it doesn't matter *how* much threat you've generated, since Taunts generate the exact amount of threat needed to pull threat from the current target at the moment of usage. If the tank *is* using their AoE taunt to pull off of you, guess what, it's actually *better* if you generate more TPS because the tank will end up generating *more* threat via their Taunt if/when they use it.
Either way, it doesn't matter because pretty much anything the tank does will generate enough threat to pull off of you: overhealing doesn't count (so, unless the tank actually just took 21k damage in 15 seconds, you didn't actually generate 21k healing worth of actual healing) and healing threat is both discounted (50% base and further 85% as a Sage) *and* diluted (so you're only ever generating the listed amount *total* across all targets in combat). If a healer *ever* pulls threat, it's not because the healer is amazing: it's because the tank and DPS both ignored a target for an extended period of time and they both deal miserable damage.
As an example, assuming my Shadow tank actually *needed* that much healing continuously and I wasn't using any of my self heals (since my healing threat is actually worth more than yours thanks to my tanking stance), if I hit a target with a single Force Breach (my worst threat generator at just ~2k threat per use), it would take you (with your 600 TPS distributed across 9 targets for only 66.67 TPS individually) 39 seconds for you to steal aggro from me (2k threat means 2.6k threat required to pull threat, 2600 / 66.67 = 39). And that's if I hit it once with my weakest threat generator and then proceeded to ignore it completely. That entire trash pack is generally dead within 10 seconds.
Seriously, if you actually understood threat, you would realize that healers *never* need Guard: they simply don't generate enough threat in *any* realistic situation to justify it (you can concoct situations that are completely unrealistic and predicated upon multitudinous levels of stupidity on everyone *except* for the healer who is played to absolute perfection). The only situation where I might even *dream* about putting Guard on a healer is on p2 of Terror From Beyond, where all damage against the boss itself is reduced by some monumental quantity (it's either 99% or 90%, something like that) such that the threat generated by attacks is so miniscule that a healer *might* actually steal threat somehow (even then, I don't bother with it on my guild runs and a healer has never stolen threat from me).