Stinger's unreliable, sure, but at lv 30 it's ~15% chance to roll that +20% to damage. I think the risk/reward ratio there on all your attacks is enough to be worth it over... whatever Rush (S) at lv 30 is for regular attacks. It's really the possibility that it can apply to your bursts that does it for me. The elemental advantage restriction doesn't bother me since I switch teams depending on element. Also, since the comparison is picking from among the different union event weapons, it's implied that you're not so tied down to a single element.

Now if you're a fire main, you'll FLB a couple of Wrath bows eventually anyway. The context here changes a bit to being annoyed at how whoever handles events hates fire. The very noticable stretches of no fire events aside, what you do get outside of old advent FLB stuff isn't all that great, relatively speaking. Having your first union event weapon's FLB being Rush is adding another insult onto the pile.