In my experience of writing articles and reviews for different publications, I didn’t follow a job description and I did my writing in a time frame that suited me. I wasn’t a career writer.
There were no specified expectations of me, so I got experience in freelance writing as I wrote articles and reviewed movies I wanted to.
I learnt that focussing on one writing opportunity at a time would have been better. That’s learning from my experience.
There may be factors preventing one from focusing. One may feel he misses out on something more in writing if focussing only on one publisher.
But you can always move onto another publisher when you’re satisfied with writing for the publisher you’re with. Move onto the next, if the opportunity presents or is a good opportunity. Remember that not all opportunities are equal and one may be better to you than another.
There’s a sense of achievement at knowing you’ve produced articles for a certain publisher or reviews for a certain publisher in a specified time frame. At the end, you’ve done focussed work every week for years.