The trouble is that by simply looking at the pads from a rider's vantage point not removing the wheel is trouble inviting. Pads may wear out unevenly. Without removing the wheel it may look that there is enough of braking material on the pads but in reality the pads may be completely worn out in the areas where you don't see.
Here comes a surprise. The end of the spring that spreads out the pads may not have enough of the "step" left to stay put as with newish pads and will simply escape, slide in the rotor area and will start making clicking noises. If this happens on a trail, you are in trouble.
Dislodged spring "whisker" is indicated by the arrow on the picture (of course this picture is an imitation and I placed the spring in a crooked way intentionally, to illustrate my point. Observe only the part of the spring indicated by the arrow, in the actual brake it will rub the rotor and the brake is compromised).
