The biggest benefit for me is the access to lounges and the ease of resolving problems or changing flights.
I use 6-8 flights per week, all relatively short sectors up to 4 hours, so the lounges are frequently the only places I can relax, take a shower, and catch up on paperwork. My stay at the destination normally doesn't justify a hotel so a comfortable lounge is a haven for me.
I have found that as a member of several FF programs I get a better level of service from airline staff. That may not be just, but it's the way it goes here. I can usually change flights very easily when I need to, and the staff make every effort to meet my needs. These two factors more than justify FF programs for me.
Upgrades are nice when they happen, but I'm not going to make a song and dance about that, my flights aren't long enough for that to be a factor. Some programs allow me to transfer free tickets to my wife, some give discounts on hotels, some give me cuddly toys!! Fun, but not that important for me.
It depends on what you need, I suppose.
Tchau