This change can cause issues with matrix builds. The description set in the post-build step for each configuration will be concatenated for the matrix parent job. This could be desirable in some cases, but in cases where the same description is set in all configurations (e.g. version number), this gives ugly results.
I disagree with this request. I think if you want to append to the build description, you should have to do so explicitly by referencing the existing build description via the DESCRIPTION_SETTER_DESCRIPTION environment variable in your new description. That way, you would have both options. As it is, there is no option to replace the existing description.