Q:

Private or public agenda? 

Question
Question: at work is your agenda private or public?  E.g., can colleagues see your agenda (subject + location) within, for instance, Outlook? 
Help request
What arguments can I give to higher management or to colleagues to open up their agenda? 
Background:
At the work (2000 people) of a friend everyone has a open agenda, even the CEO.  While at my work (12.000 people) by default everyone has their agenda on private mode.  Because of this it is often really hard to find a time slot for a meeting which suits for all stakeholders when you want to plan a meeting. 
As an employee this bothers me a lot since I it takes a lot of time to find a suitable time slot.  I will have to contact people by phone, go to their desk etc.  simply to check if they are actually available. 
When people's agenda would be open, subject + location of the meeting would be visible.  It would be a lot easier to check if people are available, and to minimize disturbance for the whole group.  I would be able to make an assessment based on everyone's agenda.  When for instance, one stakeholder would have a 1-on-1 meeting, I would only have to ask him, if it is possible to change his meeting.  Or when someone simply stated.  
Of course you could reason that people also have private appointments or they have company sensitive meetings.  In that case, at least in Outlook, people can still mark them as private. 

A:

Can you specify why making the agenda ("subject of the meeting") public will help you find the free time easily? 
The answer to your question is: it does not matter.  

What arguments can I give to higher management or to colleagues to open up their agenda? 

None, you don't need to. 

Because of this it is often really hard to find a time slot for a meeting which suits for all stakeholders when you want to plan a meeting. 

Why is that?  If I understand correctly, you'll see the time-slot is occupied or not irrespective of the setting.  In case of an open (shared) settings - you get to see the subject of the meeting / appointment, in case of a closed one (not shared), you don't get to see the subject.  How this affects the ability to find "free slots"? 

simply to check if they are actually available

If someone is busy, they are busy.  What keeps them busy is none of your business unless you are managing them.  
On the other hand, if you know that people are making "fake" meeting appointments possibly to avoid important discussion which are expected of them (or their roles), then there's a deeper dysfunction that has crept into the organization which needs separate inspection.  Opening up the agenda is not going to solve it.