I'll be honest with you, I don't like it - but it's necessary in web development to have at least some rudimentary front end skills even if you are a back end developer. Firstly because you will find yourself wading through HTML and JS at some point even if you are a backend developer, and secondly because you won't always have the relevant people with the relevant skills at hand. Right now I am working on kickstarting an internal system for a shoestring startup, and I am currently the only developer on the team.
The important thing at the end of the day is to be product minded, not technology minded - so whether you learn more of one or the other should really be lead by what is required of you at the time. Right now I'm learning far more front end than back end while I working on integrating a new VueJS based interface on their legacy system, and then later I will have my head buried deep in either Elixir or Go while I replace the backend.
The notion that one should learn one thing more than other is really only relevant when you're talking about curiosity and interest, or when you are new to an area of development and you need some skills to get your foot in the door. After that, it's really a case of transferring skills between technologies you've learned as and when you devise solutions to solve problems.
P.S. I hate that I can't type in Markdown in Hashnode any more ;P
Cliff Rowley
Thinker, Tinkererer, Dork.