SEO is important to get readers. But if your writing is to obviously geared towards search engines you will quickly lose readers. When you write for a keyword phrase, you need to use the phrase enough so that it's recognized as the topic but not so much that your readers notice. If you have the same phrase repeated in a sentence, that's too much. More than twice in a paragraph is too much.
Spin Rewriter integrates with almost any SEO software. This allows you to post high quality articles quickly and easily. The only drawback is that you only get something like 300 API requests every 24 hours. You can quickly burn through this if you’re doing multiple campaigns. The developer does not currently offer a plan for heavy API users but maybe they will in the future.