This piece argues that they are: Why Calendars are More Effective Than To Do Lists.
I think there are definite benefits to using calendars over to do lists. For example, when you need to work with other people. Scheduling time makes sure people commit to working on something and getting it done. Calendars are also great for when you need to give yourself a deadline.
I think todo lists are better than calendars when you aren’t sure how long it will take to do a task. Calendars aren’t great if you spend a significant amount of time planning to do things versus actually doing them. (Although you can procrastinate the same way using todo lists.)
One way of merging calendars with todo lists is to work in sprints of 1 to 2 to 4 weeks, like agile developers do. At the start of a sprint, go over your todo list and prioritize and size your tasks. Then fill up the sprint period with the tasks you can get done in that time. Then you can schedule them on your calendar to remind yourself to get them done. If you have things blocking you that day, plan to resolve them by eliminating the blocker.