I think it's good to have a wide range of knowledge, having full stack experience broadens your job market as well as being able to create pretty much any type of application. I have created and administered databases, web servers, written thousands of stored procs, written front ends and backends in numerous different languages and platforms. Having a wide array of knowledge is very empowering.

I prefer to use whatever tool that makes the most sense for the job. For something simple like a landing page, jQuery or vanilla JS would do the trick. If you're working on something that would require multiple screens and data entry, a framework like React or Angular would make more sense. I don't see a need to be add complication to something unless it's warranted. Also a lot of people have legacy jQuery apps that would require too much time to convert to a new platform so they're stuck working with it. I fall into this category and I'm sure a lot of other people do as well