For AoE damage arriving on a timer, such as Infest on Lich King, take a look at your own AoE healing toolkit and plan around your own cooldowns. Continuing with Infest as our example, it has a 20 second cooldown and a 2 second cast time. Light of Dawn is on a 30 second cooldown that can come down to 20 seconds with the Glyph. This makes LoD usable for every Infest and, at least on normal modes, it is an easy way to heal the debuf off the whole raid – if positioning is in your favor.
(Since patch 4.0.1 and changes to the Glyph of Light of Dawn, this no longer applies)