Wouldn't you think that the cook has to be vastly more knowledgeable and skilled? Yet for some reason we don't tip the people who actually cook and prepare our yummy meal but we do tip the people who take our orders and bring our drinks. That has always boggled my mind.
I've worked part time in the restaurant industry for over 15 years (it's my evening job at the moment). I also have a culinary degree, even though I never pursued being a chef as my primary profession.
Line cooks need to be good multi-taskers, and they need to know how to follow a recipe and cook to the proper temperature. That's it. Having said that, most of the people I've worked with as line cooks are considerably more than that. They are passionate about food in general, enjoy their work, enjoy putting out good food, and enjoy the people they work with. And I would agree that they should be tipped out at the end of the night. It's hard work when it's busy, trust me I know. I've worked hours on end without being able to take a 5 minute break.
I do it because I enjoy food, enjoy making people happy, and enjoy the challenge. The only time I get tipped out is when I help the FOH by running food to tables and such. Rarely happens.