Yes it is.
>Is it worth playing if I will most likely never buy the game
Yes, mind you the base game is a bit rough, possibly the weakest part of the game. But once you get to Heavensward expansion. From there, its a giant leap uphill. They took alot of the flaws with Relm Reborn and ironed them out. Better dungeons , story, and boss fights (Trials). This is from 50 to 60. From here alot classes really pick up in terms of abilites.
Stormblood is a bit on the weaker side storywise, but the dungeons and trials are even better. Good chunk of those trials are fighting previous FF series bosses. Alot of classes get better from there.
Past that trial, you're missing out on the (mostly) best expansion yet. Best story out them all, really ties things together. Not to mention, my favorite trials are all here. Its about to end, with a new one later this year. Classes feel really complete at this point til the next expansion.
Criticisms I have is the normal dungeons and trials are lacking in terms of diffculty. Great if you go through them with other new players. Mobs are all pretty similar in how they behave, they're just in the way. Wish they where more unique, maybe more special abilites. Somethings are a bit annoying to grind, but not the worst. Ive already mentioned the base game issues.
>because having to pay every single month to play something you already paid for is the most retarded fucking thing I've ever heard of
Datacenters cost money to run, helps with the costant develeopement costs to. I don't agree with it otherwise, not to mention compared to other MMOs, you only need to by the latest expansion, which is 40$. Correct me if I am wrong.
But if you don't normally like MMOs, you may not like it. Isn't guenteed but just a warning. I would just do the free trial and come up with your own opinion.