On-use and proc relics really aren't worth a damn in my experience (PT tank). You're counting on them to be useful/available when you need them the most, and that's just not the case. The tank should be in control of when he needs a boost, not leaving it to chance.
The logic between those 2 statements is completely contradictory. The tank controls when to use the use relics, so the tank has the ability to specifically activate the ability when needed. The proc has a 100% chance to activate when you successfully shield, which, unless you're not stacking Shield whatsoever, is going to happen within 1-2 GCDs of the ICD coming off. The proc relic may be "random", but it's got a good enough uptime on a short enough ICD that it more than offsets it.
The only advantage to the passive relics is that they are just that: passive. The proc relic has the best mean contribution even though it's only active for ~30% of the time, and the use relics have the most controllable/burst damage resistant nature (because they're functionally an extra CD). The passives are only best if you're largely unwilling to learn fights and then react to the changing situation and varying damage levels virtually every boss operates with now as well as unwilling to cede to the mathematical superiority of the proc relic (which, regardless of how you feel about randomness, provide more mitigation than the passive relics do).