This is the talent row that always fields the most questions. Back in Legion and early seasons of BFA, many Holy Paladins ran Beacon of Virtue to handle all the AoE damage that seemed too rough to heal through without it. Fortunately now with Glimmer of Light, we get a massive amount of passive healing for 0 mana cost, making this the most efficient option for Mythic+ healing. Not only will you gain huge passive tank healing from keeping your default beacon, but you'll also get a significant amount of free AoE healing from Glimmer itself. The key to running without virtue is knowing all the dangerous pulls and bosses and how to prepare for the aoe damage in those spots (this is gone over in more depth in the Dungeons section).
Should you ever use beacon of virtue?
Virtue is a strong talent choice if you're playing with pugs or less coordinated groups (early in the expansion when everyone is still learning mechanics). It also is a great crutch for newer players who are still learning how to get the most healing out of Holy Paladins. That said, once you feel comfortable enough on the spec and people start to take less avoidable damage, you'll definitely want to transition to using Glimmer of Light as it is far more efficient, giving more throughput for 0 GCDs or mana.