I tested the Dread Guard proc healing relic on my Assassin tank against the combat dummy and over a 5 minute fight it healed me for a massive 6096 - I'd hardly call that substantially improving my survivability.
You need to remember that the amount the relic heals for is static. It does not scale with willpower or against total health (like other assassin self-heals). Maybe if it crits the heals are slightly better but in my opinion this type of relic is still useless (even for a healer).
I know the numbers don't sound like much, but that's only because you're dramatically over-estimating the amount of damage you're mitigating. Not because mitigation is bad or being bypassed, but because the amount of damage you're receiving is very, very small (relatively speaking). See my spreadsheet linked in an earlier post.
The proc heal relic is really, really good for assassin/shadow tanks. It contributes about 20 HPS, which is an increase of at least 15.3% on your base self-heal. Seeing as your self heal is a significant component of your overall survivability (13% to be precise), a 15.3% increase is nothing to sniff at.