If what you are doing works well for you then great. I was only pointing out that there are situations where a mini-site may be a superior choice. Each website should be designed with your primary goal for the website in mind.
If your goal is to build an authority website then your method is an excellent choice. However, if your goal is to run a limited promotion on a specific product, or to generate leads for your service business, your approach will typically yield inferior performance as compared to a well-designed mini-site.
With that said you can build landing pages that are specifically designed to act as a mini-site by removing the navigation from that page, and while it is on the same website it is made to appear as a mini-site when you arrive at that landing page. There are no hard and fast rules, just build pages that are designed to serve a specific purpose for maximum results.