Any of the above is a valid reason for inviting an outside speaker to minister at your church.
WHY GIVE AN OPEN TOPIC?
Let's relate this to the reasons for inviting an outside speaker. Both (1) and (2) imply that you have a specific topic in mind; only (3) gives room for an open topic.
What are some possible reasons for giving an open topic to an invited speaker? I could only think of one ... granting convenience to the speaker. The speaker could speak on a topic that he had delivered before or on his favourite topic. Can this be a valid reason? I don't think so.
Don't go for a popular speaker and give the speaker the freedom to speak on whatever topic because the chosen topic may not be relevant to your congregation. Giving an open topic reflects your purpose ... to merely fill up a speaking schedule ... just come, any topic will do!