Yes you are absolutely right, it depends on what you want to shoot. Speaking for myself, I don't need this kind of telephoto lens except two or three times a year, but when I'm out and about with my friends and I go for a walk or I want to escape the city, it can create some great photos.
The Tamron 70-300 also looks like a very nice lens, although I haven't had a chance to try it. Being able to reduce the aperture down to 4.5 is very advantageous for such a lens.