Quote Originally Posted by _Babble_ View Post
One more question on this topic: is it beneficial or maybe even necessary to continue to level my subclass after I hit 15 and choose a job?
Is it beneficial? Yes.
Is it necessary? No.

As the above poster said, you can get cross class skills through levelling other classes - in the case of Arcanist, that's an Eye for an Eye, which can be handy, but isn't absolutely vital, nor does it make or break you as a healer. If you enjoy playing Arcanist and want to try Scholar/ Summoner one day, by all means get it, but don't feel obligated to otherwise. Thaumaturge's Swiftcast, on the other hand, is an absolute necessity, as you will probably realise once you start doing more trials/ working towards level 50 content. I recommend getting it ASAP.