Oh yeah, it's worth it. Besides the obvious 'they make small things look bigger and make far things look closer' there are some shots that you just can't get without the tele.
I had been trying to get pictures of hares, but with the kit lens the photos always looked like a little bit of fuzz on a huge frame of grass, so the subject was lost. With the tele, I was able to remain at a distance where the hare didn't flee and get a decent shot:
I went to the FIFA U19 tournament at Commonwealth a few years back, and took several pictures, all which disappointed me because the players looked like ants on the pitch. Sometimes a tele is the only way to get a good shot when it is impossible to move physically closer to your subject.