By design, the comment field is meant to be an instantly available and simple way of adding additional information to the form. For more complex scenarios (like conditional behavior of the field) you'd need to use multiple lines of text field and then play with its behavior. You'll achieve the same result.
From our perspective, it's important to make the platform as useful as possible while keeping it lean (every additional feature needs to be maintained, be compatible between versions, etc.). Therefore, since the desired outcome is fairly easily achievable in other ways, it would be hard to justify implementing an additional feature just for the comment field.