Per the PMBOK (r) , the Project Management Body of Knowledge, Project Schedule uses the most number of inputs and is used by the most number of processes or documents, either directly or indirectly. Also, the most number of processes fall under the Planning process group. (see Pages 61 and 173 of PMBOK(r) Edition 5).
Indeed, the schedule relating to so many other processes is also true in a real life project. So much information can go into the preparation of schedule and can be executed and be tracked by it. The process groups of Execution, Monitoring & Control, and Closeout are all based on the foundation of a well planned schedule.
As for PMP format and preparing for the exam, a candidate who does not understand scheduling well, will need to do a lot more memorization than someone who understands it. This is because of the overwhelming number of processes, inputs and outputs that are presented in exam theory, and the fact that many of them either directly or indirectly go through the project schedule. Seeing how these processes fit to each other within the big picture is considerably easier, if one has a good understanding of schedule. There are many exam preparation practice tests out there, which have questions that ask about all processes and schedule as well. However, I believe that the project schedule and how it fits into the whole picture could be asked in more detail, with more detailed solutions.
I wrote a test book for it. It is called “Two Full Length tests for PMP Exam” and can be found on Amazon.com. It has greater focus in Planning, and in particular, schedule. It has a sample schedule of 16 lines, which is a simplified version of a real life project, and many questions are derived from this same schedule, that relate to each other. Solving questions in different subjects by looking at same schedule is helpful to see how they fit into each other.
The portion of questions about scheduling in an actual PMP exam, may not be very high. Per the publicly available information on PMI(r) site, the percentage of questions on planning is 24%, which can be found at the pmi.org website. Out of this 24%, the portion dedicated specifically to scheduling cannot be known and only be determined by PMI(r) for each test. So in actuality we are not necessarily talking about a very big percentage of questions here, but this does not alter the importance of understanding the schedule within the whole project management concept.