If you switch lenses, it's impossible to avoid dust getting inside. The smallest particle of dust will look like an immense glob on a large print. Usually, the blower bulb is enough to dislodge these, though sometimes all it does is move them around. Also, as bdjohns noted, some contaminants are sticky and won't be dislodged by the blower. Sensors are delicate and can be damaged by the force of air from a can of compressed air. Also, the propellants used in the compressed air cans can cause chemical damage to the sensor.