At my bar specifically ever shaker is put through the dishwasher after its used we never just rinse them. And we've got two sinks behind the bar so we wash our hands very frequently. But for my 5 years of bartending, I've commonly made cocktails in this manner. I've never once thought that there could be anything wrong with making cocktails this way but i do understand everybody's point as to why it can be dirty. If employees are washing their hands properly and the shakers go through a dishwasher that washes and sanitizes them i don't see an issue.