Without having seen the backside of the rotor or the thickness of the brake pads themselves, from what I can see I'd say your brakes are far from needing changed, and especially not due to any little bit of surface rust which is present.
For the rust which is present, it is easily explained. When brakes are used they get very hot. Except for a few exterior coatings which prevent this, most coatings/paint heat up and peel off, leaving a nice fresh surface of bare metal behind (even more so if there's no coating in the first place). As you probably well know, bare iron or steel will rust very easily. The cast iron of this rotor is no exception.
The apparent smoothness of the side of your rotor we can see leads me to believe you have plenty of pad left at least on this side (the inside may be a different story). Usually when the brake pads get shallow, they start digging into the rotor and will cause all kinds of valleys and ridges. They may be large or small, but you'll definitely start seeing relief on the rotor surface (not always, but in a VAST majority of the cases). As smooth as this is, I'm not seeing replacement being an issue. As someone else stated somewhere, if you're at 5mm thickness of the pad, consider changing it soon. If you're less than that, you really need to get it changed for safety reasons.
From seeing what we can, I'd not have an issue using these brakes. They look in fine shape.