Hello,
I suspect you mean a (smart) shape button on the master slide (maybe a translation problem), just mention this to avoid confusion for other users. Maybe you misunderstand: such a shape button is not a 'placeholder' at all, it is a fully functional interactive object that will be available on each slide based on that master slide. A typical example is a Next button: if you have it on the master slide, and you check the option to pause, that Next button will appear on each slide and will be pausing that slide at its end, whatever its duration. This will avoid the necessity to have an instance of that button on each slide. The drawback is that the Next button will appear on each slide right from the start, that it will always have the same 'look' and the same states. If you want to be able to give the shape button a different look on each slide, you need to have a shape button per slide. Another drawback is that objects on master slides do not have an ID: that means you cannot control them (like hiding/showing or disabling/enabling) by an action.
The placeholders for titles, text captions, images are something totally different: if you don't want to replace them by real text, real image, they will not show up in the project, will be ignored. You cannot have a placeholder for an interactive object, only for static objects.
I have several blog posts about shape buttons, here is one of them:
Why I like Shape Buttons - Captivate 6! - Captivate blog
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