Don't confuse concept art with some "illustrations" which just show something.
concept art or references in general can be a great help.
the purpose of concept art is to nail down the design and the proportions of assets. Making pixel pieces usually takes longer than sketching some quick lines on paper.
Most concept art also is made out of a flat angle (frontview, sideview) without any perspective, to help you to effectively measure out proportional relationships and to draw the asset then more correct in a game projection.
Effectively concept art don't has to look pretty, it has to be more like your plan and to make you artistic choices easier.
Good concept art will lead to a better and faster result, because all steps of the process are much clearer and you just have to focus on drawing the thing.
All the big design choices were already made during the creation of the concept art.
If the concept style and the pixel style are in line and you can effectively do all the details the concept art prtovides with your pixels, there shouldn't be any problem.
If the style of the game is clear to the concept artist and the pixel artist the result will look the same. If you are doing a quick paper sketch for something you will pixel later, it's basically your concept art.
Depending on communication and result orientation, a concept artist can be a huge relief or a huge hindrance or something inbetween.
