A competent writer can be had regardless of whether that writer is ESL or ENL. The main qualification the writer must have is his ability to properly research and discuss the assigned project. It will help if the writer actually has a background in the field he is writing about since that will lessen the chances of miscommunication with the client.
The only difference that I can see between the two writers is the way that they express themselves in English. I have been told that my capacity to use the English language is proficient enough, but still obviously ESL. Since I never claimed to be an ENL writer in the first place, I see no reason for my English ranking to matter. An active ESL writer who can meet the writing demands of an ENL student will not receive complaints from the client, more so if the writer assigned is an ENL.
I think that the writer competency zeroes in on one particular requirement, that of his expertise and ability to actually write about the topic assigned. As @a1writer often notes, it is important to hire a writer with a relevant background but, doesn't on the job experience and collected knowledge count as well? If a writer has been at this job for decades on end, written papers on a massive number of topics, gained knowledge and experience through those practices, then I believe that it makes him or her competent for the job.