single-climate worlds

Hope it's not a dumb question from a sad SF nerd again...
Planets with a major environment-type are common in science fiction (eg.
Tatooine: desert world, Dagobah: swamp world). Is it plausible for
(habitable) worlds to have one major type of environment, or is Earth
with many types of environment a more realistic model? Mars may be a
uniformly cold-desert planet (similar to the Tierra de Fuego?)--does it
have much environmental variation compared to Earth?
Note that I'm talking about habitable planets: many not-very-habitable
planets/moons like the Moon, Mercury, etc. are quite uniform if I'm not
mistaken.

Seb