Invocation- This talent is best used for encounters requiring a quick burn of a particular mob or mechanic. However, due to movement/getting interrupted by various mechanics, this talent is dangerous. You must complete the Evocation cast in order to get the damage increase, so if you’re forced to move before the time finishes, or get interrupted, then you completely lose out on the damage bonus.
Invocation also reduces the CD to 10 seconds, so if something goes wrong you can try again soon. The idea is to use it every 40 seconds to keep the damage buff up.
While up for debate, I'd say it's best for fights when surges of damage are more important, although probably not a very good Fire talent overall.