Leviathan weapon might not be better than Allagan for some (or all) classes, but just like Allagan vs AF2 gear it wasn't ever a deal breaker (maybe on Twin for some, no idea). So being able to get Leviathan HM weapon should make Leviathan EM more doable to get the i100, which in turn will make Second Coil more doable I would imagine. For whatever reason, I just never wanted to try to learn Twin. Probably because of how hard people made it sound and I remember how much time (10+ hours at least each) I had to put into Garuda, Titan, and Ifrit to "get good" at them and Twin sounded like it was on a whole other level.
I preferred to spend time leveling other classes, doing EM Primals/T1-4, and grabbing as much myth/phil/drops to secure full i70-i90 gear for all my classes. However, since I pugged coil forever, I spent a LOT of time with groups failing in T4 or taking longer than usual on T1. I could only imagine if I had a static early on in 2.1, I probably would have Twin on farm. But basically, I was just farming myth early on in 2.1, then at some point I decided to try Garuda, then Titan, and then finally Ifrit. I just took too long to learn stuff and get good. I also did not play as much as I do now until like, maybe start of February. Before then, I would maybe play an hour every few nights and then finish hitting myth cap on Saturday or Sunday and bail. Now I play like 4 hours each night and more on weekends.