One of the most popular solutions nowadays to get likes and subscriptions on a website by suddenly popping up a modal dialog with a request to like and share the page, subscribe to a newsletter, or using some sale offers.
Such trick might drive subscriptions and likes, but it is a real user experience killer. It is simply annoying. Annoying because the modal is unexpected and wasn't triggered by the user or visitor. The modal auto triggering is called to block the workflow in order to make a fundamental decision or to display a critical error, which is not the case for subscription forms or ads. It is a very bad practice to auto trigger the overlaying dialog in term to serve the marketing goal. The correct way is to serve people with the best user experiences. If the goal of business is to annoy people, rather than helping to achieve greater results, it is a simply unethical business.
Suddenly popping up dialog destructs the visitor from his goal appearing with a timing delay in an unexpected manner. It obstructs the visitor from reading the content, watching a video or performing an action. The immediate will is to close the dialog and in many cases, the visitor clicks on "Like", "Share", "Subscribe" buttons because they are designed as a call to action - the most visible element in the modal. The comparison for such abstraction could be as if someone walks on the street and a stranger steps into the path by blocking the further away. The reaction will not be to "Like" and "Share", but to stress.
As a designer, you have to take the responsibility, disallow the misuse of the element, and ensure the smooth flow for the user. There are better, non-intrusive ways to advertise or collect visitor's information, of which creative person should think. It is way too easy to isolate the user and stress him into an unwanted action.
Reflections on founding principles of good design.