The internal osscilator can also be calibrated through a CPU register (OSCCAL) and as such can be made highly accurate and suitable also for clock sensitive applications.
It depends on what you mean by highly accurate. According to the datasheet, the internal oscillator calibration accuracy can be improved from plus/minus 10% to plus/minus 1%. A clock running with 1% accuracy could have an error of almost 90 seconds per day. Arduino running from a crystal is accurate to a few seconds per day.