I thought it was common knowledge that the flu shot was primarily intended for 6 month to 4 year olds and those that are 50 years and older. It isn't really that necessary for those in between but it might help with some strains. High dose shots are actually harmful for those under 64 years.
So I could see why many people don't get them. The odds of you being protected from flu shots given the multiple types is probably the same as you picking up the common flu when you don't get the shot (assuming you are abiding by most standard sanitary considerations).
If you are in a business where your PTO is the same as your sick time off, you should always get the flu shot to minimize the risk of losing PTO. If your sick days off are separate from PTO, well then I suppose the only incentive is being nice to friends and family.
More information about formatting options