IMHO:

* A label identifies what a field is for, even after text is placed inside of it.

* A placeholder gives an example of field content.

These are somewhat opposed. A further way of looking at it is to imagine a long form fully filled out -- how do you verify, as a user, if you've filled it out correctly if the labels are no longer visible (because you've filled in the field).

On mobile, for short well known forms: e.g. password forms; there might be an argument. But I would certainly rather not see this as the default.

 

~Matt Walker