Learning to say "thank you" isn't just polite, but it's an excellent way to rally and encourage your team.
Sometimes managers get sidetracked and fear being perceived as "weak" if they're too "nice". Such attitudes are terrible to have, and should be thrown out the door: we are first and foremost all human, and a good thank you goes a long way to making people feel appreciated and wanted after a job well done. A sense of acknowledgement goes a long way to inspiring loyalty. And it's simply a nice thing to do.
Of course this doesn't mean a continual stream of unwarranted or fake praise. While continual praise is important, it also needs to be genuine, as otherwise your efforts become robotic.
In the movie Hancock, Will Smith plays a dysfunctional superhero who is eventually taught to use his powers in a productive manner. One of the things he learns is the need to show appreciation in the form of "good job", something that we should all learn to remember.