I am currently working at my third coffee bar, but my first where I'm allowed to collect tips. This is obviously a nice little perk. There are the financial benefits, but the tangible appreciation is every bit as nice. It's one thing to say "Hey, that drink was pretty good"; It's another thing altogether to let George Washington speak on your behalf.
There is still the question of whether one needs to tip their barista. After all, they make above minimum wage, often twice as much as servers at restaurants. Few would argue that waiters and waitresses don't deserve tips. In fact, it's a necessary addendum to the price of your meal. They make jack shit. They take care of you and, ideally, are pleasant company when you want them to be, and out of the way when you'd rather they vanish.
A barista is not totally unlike that, but the more obvious comparison, of course, is to the bartender. Hell, "barista" translates to bartender in Italian, even. And the jobs are virtually the same. The talk to you as much or as little as you'd like, are knowledgeable about the product, and get you your damn drink. If you just get a beer or a coffee, there's probably not much need to tip. After all, most anyone can pour liquid in to a glass (although there is a right way to do it). But say you purchase a mixed drink, or in the barista's case, something that requires more than just coffee. In that case, tip. Please.
Again, the money helps. Baristi are not wealthy people. Trust me. We make more than waiters, but still less than ten dollars an hour. And it is skilled labor. If you don't believe me, walk behind the counter, and try to pull your own shots, and steam the milk. You will make a mess. If, somehow, you manage to make a drink as well, it will be awful. A decent barista makes all this look easy -- and after a while, it gets to be -- but don't think that makes the task altogether simple.
Finally, in the words of one of my better tipping customers, "There's a culture here. Not everyone knows it, but we do." He's right. By tipping, you're filling your role in said culture. You feel good for doing something nice (even though, at most, you're typically dropping an extra buck); And trust me, the barista feels even better. There's no satisfaction like knowing you're appreciated, that the work you do is valued. So express that appreciation. Tell them that you like the drink -- if in fact you do -- and why. They'll love you all the more for your knowledge.
And the money is nice, too.