As Nathan pointed out, this angle can work for many reasons. For example, it can be flattering to a full face, as he mentioned. In an outdoor portrait, it can bring catchlights into eyes that might not otherwise be there. In the case of a child, or even a puppy, you might want to convey a sense of vulnerbility, you want the sense of how "small" they are, so that angle will work nicely for that. Sometimes, a slightly higher angle will cause the subject to open their eyes a bit wider than they would otherwise have opened them. Every case is situational and it depends on what you are trying to achieve. The rule of thumb, that portraits should be taken at eye level, is just a rule that is meant to be broken, situationally and for a reason. I say all of that without even CONSIDERING depth of field, only in reference to the angle of the photographer to the subject. As far as the DOF goes, that is often an artistic choice.