I agree.
I agree with all the reasons mentioned by Yana.
Here is my take on mixing both
We learn the differn styles from different publishers. I was able to condense and write when I wrote for "the shortform"
"The Six word Photo story" helped my improve my photography skills and get creative in linking/integrating photography with writing
Writing for Illumination helped become patient with myself and write longer articles.
A couple of other Publishers helped me improve my discipline to write, get out of writers Block when I participated in their challenge.

Considering all the above I would do both