To make it simple for everyone. The only usage of Agony glyph is to simplify cyclemanagement. It not a major dps increase.
SL / immolation are the glyphs to pick if you'r after damage increase. If you feel the agony one simplifying things for it then go for it.
I've seen som many locks that pick only damage glyphs but suck at keeping a good cycle up, the prio are:
If your a player that find cycles hard, you pick runes that make it easier for you, agony, maybe even UA
Then and only when you'r good at your cycle you start stacking glyphs that increase damage.
Annoying that some players I've seen don't understand that no matter what glyphs they got, if they got a glitchy cycle it wont give good damage no matter what.