Of course, there are two more parts to the equation: 1) The back of the house. The kitchen can have a lot to do with what most people think of as their restaurants service, such as how fast their food comes out, whether it's properly made, etc; a good server will affect your experience of this, offering a drink, perhaps, if your food is slow or comping an improperly made dish. But people often tip based as much or more on what the BOH does than what the FOH does. And that's fine, since the FOH should be tipping the BOH in most cases. 2) The management. Servers are accountable to customers through tips, but they're also accountable to their employers. Or at least they should be. Some managers don't make their staff accountable. As long as they punch in every day, they keep their jobs.
Yeah, I had a serious talk with a friend at dinner one night when I still lived in NYC. It was a bad night in the restaurant, the BOH was clearly screwing up, getting orders wrong for multiple servers, some dishes not coming out at all, there was a bit of screaming overheard... My friend was all, "I am not tipping, this is ridiculous!" But our server tried to do whatever they could, made sure bread and water were refilled, kept coming by to apologize, kept going to the manager and bartender to see if she could offer free drinks (idiot manager obviously said no)...working her ass off trying to make sure our dinner out was not a total disaster. I explained this to my friend, and she looked at me and said, "Wow, I never thought of it that way...you're right, it's not her fault."
Now, I wouldn't necessarily return to a place that can't get it's shit together, but neither am I going to fault a server that is doing whatever they can to salvage my dining experience.
But most folks just don't get that...and it's a shame.
My mom was a fan of the 2 cent tip to get the point across for bad service. Her view was that she did not want them to think that she forgot to tip at all, she wanted them to get the point that the service she received was only worth 2 cents.