In general, I would say that ghost writing does not constitute plagiarism and using one is not an academically dishonest. A scientific writer who is not an expert in a particular field can convert research notes into a manuscript. These words are not the contribution to the field, but rather the research they encompass. While converting the notes to prose could be viewed as worthy of authorship, I think it is also reasonable to say that it is not. If the writer and the researcher agree that the writers assistance is not worthy of authorship, then there is no problem.
While I do not use a ghost writer, I see this as no different then employing a programmer to write code to control my experiment, an RA to collect the data from the experiment, a graphic designer to create the figures, and a statistician to run the statistics, all of which I do do to varying degrees. To me the contribution is the design of the experiment and the analysis and interpretation of the results. I acknowledge these individuals to varying degrees in the manuscripts, but rarely give authorship.
The ICMJE guidelines for authorship are
Authorship credit should be based on 1) substantial contributions to conception and design, acquisition of data, or analysis and interpretation of data; 2) drafting the article or revising it critically for important intellectual content; and 3) final approval of the version to be published. Authors should meet conditions 1, 2, and 3.
A ghost writer does not meet conditions 1 or 3.