A lot has been discussed on this very forum about this dichotomy, and I feel adding more is still justified because the mighty debate continues as people from top up to those on the ground continue to revisit this position.
There can be no doubt that an ESL/EFL writer would present some obvious deviations in their spoken/written verity of English from ENLs. However, within ENLs, I have discussed it quite a few times already, there are a number of varieties that are different from each other: Even British vernacular is different from that spoken in American. Within US, there is this North and South-ness of English.
Australia and New Zealand stand to be distinct versions; let's, at this point, drop the pidgins and creoles of English.
There is tremendous linguistic variation within the ENL concept, and it is now well established that not just language but also cultural tones are better described on a continuum than something as a fixed concept.
Not just English, every single language and culture on this planet continue to evolve as long as people represent these.
This is well justified that a student from an ENL background (again, which ENL background?) is logically suited to be assisted by an ENL writer who is well-versed in their version of the language: use of certain words, articles, etc. This market positioning is most probably every ENL writer's niche. With their competence of language and research, their assistance to an ESL/ENL writer is probably equally rewarding.
However, let's change the ENL perspective and see the world through a different lens.
Middle class populations from India and Pakistan (leaving aside China) make an enormous number to the world's population so much so that some of the universities in US and the UK are dominated by students from these three countries. Google is our friend here.
Original research and development and knowledge generation is no longer dominated by the West. Major publication houses feel increased pressures to situate research produced in other countries. While English remains at the center of the development, certain discounts are being made.
I started off as a writer to primarily serve our ESL/EFL students. However, I started assisting ENL students who came to me voluntarily. Though I worked much harder to do their papers in the beginning, they were more than satisfied. Currently, I serve MORE ENL students than ESL/EFLs. I am convinced that my written English is different from an ENL writer, but then there comes in the role of the quality of research and academic jargon which, most probably, pretty much put aside the intricacies of language (something I have learned from my experience).
Probably, I would conclude, the dynamics of our industry are far more complex than considering the command/use of English, which certainly counts.
