Burst isn't so bad, it's just situational. It's great for burning a mob due to it's low MP cost and slightly higher damage than Thundaga (even on crits). When doing Coincounter in AV you're going to want to burn it with thunder spells when you get at around ~700 mp you're going to want to do Thunder > Thundara > convert parsimony excrutiate > Burst. After this you'll be at a little higher than the 2200 mp you get from convert.

My BLM is nowhere near optimized i just use NQ accessories, mogstaff, and BLM JSE and it still crits around 1.4k (on Coincounter.) On things like Chimera I have found it still to be relatively useful for the burn phase because of it's low MP cost. But even then it's not really game breaking. Just like every other 15min, it's useful just situational. For example you're not going to want to pop benediction if you're at risk of pulling threat while fighting Chimera because then you'll wipe you're whole group lol