Just as important as back injuries actually caused by poor office chair ergonomics, are the injuries that occur outside of work, but are made worse by poor office chair ergonomics.
Hours of sitting is the key here. When you sit, all of the weight from your upper body is transferred from your lumbar spine into your pelvis. If you have a lower back injury, or are suffering from lower back pain, constantly loading up the lumbar spine through sitting may make your injury worse, or slow down your recovery.
With this in mind, being able to switch between sitting in office chair and standing is a great option for those with lower back pain.
However, if a standing option is available, that is not the end of the world. It just means that setting up your workstation ergonomically is all the more important.
Something else to consider for those with pre-existing back problems, is that prolonged sitting results in a reconditioning of the core muscles. This means your lower back is less supported and more susceptible to injury.