As @WBahn pointed there are advantages and disadvantages to using asynchronous counters.
If you are only "looking" at the outputs of the flip-flops, in other words, looking with the human eye, such as in a ratemeter or frequency counter design then the glitch is not "visible". Don't say that to circuitry that is looking at all transitions or counting state changes.
If you are only "looking" at the outputs of the flip-flops, in other words, looking with the human eye, such as in a ratemeter or frequency counter design then the glitch is not "visible". Don't say that to circuitry that is looking at all transitions or counting state changes.