Can I output the Placeholder Text of a plain text matrix block? Is there a way to output the Placeholder Text of a plaintext field if there's no specific value entered to the Entry? 

(Am I missing something — should it happen automatically if there's no specific value entered?)

The docs say: "The text that will be shown if the field does't have a value" — does shown just mean in the form on the Entry form in the admin, or also on the front-end as a value of block.FIELDNAME ?
The "docs" means only on the admin form. For outputing a default text on empty fields in frontend you can use the following code in you template:

{{block.FIELDNAME | default("Placeholdertext") }}