Is it worth it for a level designer to learn how to make concept art?

I think it depends on if it's hobby or not and how much you think it will improve your maps and how much enjoyment you get out of it.

I used to sketch a little but I get much better results If I float around with the camera in my map thinking thinking and thinking. However 2 days ago I started sketching a little again, just so i'm not behind a monitor all day.

What I used to do is make a screenshot of a part of a map where I needed to continue. I would make the screenshot in such a way that on the right side of the image you could see for 30% or something a building I mapped. On the left side nothing except brushes for the floor. Then I would print it out like 10 times with 90% transparency so you hardly could see it. After that I would draw over the outlines of the building I already mapped and after that draw a building next to it from scratch (or a wall etc.). This worked really well for me cause you have guidance for scale and perspective and you can see how it adds to what you already mapped. I really recommend to try this out.

Anyway, If time is not an issue, go make concept art. Else I would look around for it.

It's probably quicker to whitebox/graybox a level, but that might not get across the vision you have in mind for the environment. Anything additional which you can contribute to the process is valuable if your team has a need for it, whether it's conceptualizing an environment, building art, scripts, whatever. That being said, you might have times at which you can communicate a vision for your level(s) using references alone. And if that's the case, I think it's probably more useful to have extra skills which can tangibly affect the end product - such as contributing to level art or scripting. I don't think it can be said one way or another if those skills will ever come into practice in your particular work experiences; maybe you'll work with a team which wants some extra concept art to show off to the public while a level is being arted up, and you'll have a chance to put your concept skills to use. Maybe in a different setting there won't be any time for this, the team will already have concepts created, and you'll be fully focused on level design/construction.

If you have an interest in painting and/or concept art, I don't see why you shouldn't have a go at it. Even some basic skills can make layout proposals and presentations more appealing; even more so if you can put your vision for a level into a format which inspires others, and provides a foundation upon which your teammates can work off of.

Try it, you'll learn some useful stuff, even if it ends up just being a little experiment.

It's certainly no waste of time to learn some 2D. And concepts artists actually use 3D more and more now. So this might be a good first step into content creation for your levels eventualy. 2D skills can be applied to texturing and some 3D skills are a good starting point to learn about making props.

There is certainly no harm in trying. I try to sketch my ideas for others even if those sketches are terrible.

New skills are always good thing to have. So yes, it is worth it. By practicing, you get better, so don't worry if your scribbles are scrapworthy only in the beginning.

More than concept art I'd go for mattepainting. Having the basic blockout set, taking a screen and pasting in some nice photos can really help. I think it is in that moment when 2d art skills can prove useful if you are working for yourself, as setting the mood and adding details is very time consuming (modelling all the props, creating fog, sprites, godrays etc). Obviously it would prove more useful in a team.

If you're aiming for mattepainting you would only need a good understanding of photoshop, so it is easier and faster than learning traditional drawing skills. I digged into it (not much), and there are plenty of tutorials despite after watching a few I concluded it is more about practice than learning/watching.