Yes, very important, though as Simon Barker correctly says you are unlikely to notice it in isolation, but in the middle of a collection of images, very noticeable, especially if its a specific light setup to reduce shadows in a dark area.
It can also have other effects as well, say you are shooting products and perfectly set everything up, including wanted shadows. Then the lights are inaccurate and not only is your product being lit inconsistently, your shadows can move about and get darker or lighter.
To put it into context. My worst experience ever of using some awful lights in a studio on a 10 hour fashion shoot meant about an extra 3 or 4 hours editing eyeballing, tweaking, analysing, eyeballing, and more tweaking a large number of images to ensure they were all balanced properly. It wasn't just as simple as brightness, I had a 5 light setup and all 5 lights were inconsistent, meaning one image was darker on the left, the next on the right. Then the next image the background was darker, the next background brighter on the left, model brighter on the right.